It can take hours so to avoid little ones having to wait patiently I would get it out before hand and load it all up. Setting up the console is easy, HDMI lead into the back of the console to the TV, the lead comes included, then a 2 part plug socket, which you connect together and then plug into the back of the Xbox on the right side ( Side note the Xbox has 2 HDMI ports I would imagine it's the same as the original Xbox one and the HDMI port you have to use is right next to the power input)
If you have to connect the controller ( one comes with it) there should be a button I don't know where in the S models but on the originals it's a silver button on the side. The controllers need 2 AA batteries or if it comes with a power pack a couple hours charging. Some phone chargers and I believe the Xbox comes with a USB lead can be plugged into a USB plug or the Xbox and can be played without batteries.
You will need a Microsoft / Xbox account you can set it up the console or on the computer. You'll need to have an email address and a phone number perhaps to help with account security and you'll set up a password, and it'll ask a couple other details name etc. It may ask you for a username think of something a little cool as if you let your children online that is what it'll come up as.
It'll during start up installation ask you for wifi details and will update. Depending on the package you will need to go to the store which is on the menu then go down a little and redeem a code which will then allow you to download the game, but if you have a disc version you will have to put it into the console and it will install and then need an update. This can take hours and probably will, especially if you have to redeem it from the online store as it depends on your internet speed but even so it can be temperamental, the plus side is if it is a digital download ( through redeeming the code on the console) then you don't have to worry about losing/ damaging the disc.
You may want to access settings on the console especially if you're allowing them online, you can change settings so that they don't have to input the account password every time they play ( or that they do if you want to monitor them that way and you input the password always ) also if they can talk/ hear other people playing you can have it set to everyone or friends only for example which means that he can talk in a " party chat" with his friends which he would need to add them through their Xbox live username ( the username I told you if they ask to choose something cool, but if they don't you could be assigned something random like EarphoneGiraffe2016 for example) I believe there is also settings for what they can do so play age restricted games, how long/ when they can play for but I'm not exactly certain of the options.
Another thing you will want to be careful of is card details they can be stored and used easily without any more info once it is stored, again I believe this can be altered in settings but if the children are that young I wouldn't risk it, so if you allow them to purchase something i would check it isn't stored. A good way to avoid this is that you can buy in different denominations from as little as £10 to various amounts higher in shops like GAME or I believe Amazon do it and give you a code. These would be redeemable through the redeem a code function on the store on the console, then when you purchase something it will take it out of that money, be careful you can't withdraw it, but if something's £30 and you have £25 preloaded you use that £25 and input your card details for the other £5 but again I don't recommend it. If you are going to allow them to play online with people around the world and their friends then you will need to pay for an Xbox live membership which is a couple £ a month I think around £40 for a years subscription( it gets cheaper per month the longer the subscription - 1,3,12 months) again you can buy this in GAME or maybe even Amazon and another reason not to have your credit card details stored as it could auto renew if you aren't careful! When you create a new account it may give you a 30 day or 14 day trial I'm not sure, but again if it has your card details it could auto renew. Without it I think he can chat with his friends through the microphone (it comes provided and plugs into the controller) but can't play with them. That's a lot of information on the Xbox itself now FIFA.
Playing FIFA for offline multiplayer you will need two controllers one comes wi the console, you will need to buy another so the DC can play together or DH and one DC, 3-4 people can play together but that's more controllers and it becomes confusing who has the ball. There are about 6 different skill levels beginner is the easiest the players just basically stand there, but if it's their first game that may be for the best, or amateur. The game should ask you a few questions to make a player profile such as skill level, controller configuration for which classic is fine and most widely used, if it's just the DC and offline two button is an option it limits the finer details of play and finesse but only uses two buttons to pass shoot/ tackle etc which for say a 7 year old especially may be ideal, these settings are all easily changed in the main menu, but if you're prompted to play a game before this it'll most likely be on Semi Pro difficulty, you can just press pause and quit out of it to the main menu.
There are several game modes:
Play now- a friendly game, this is where you play a single game of football, normally used for offline multiplayer, you choose a home and away side, one person home to the left, one away to the right. Next is the team any team can play any other apart from the ladies teams who have to play as each other, then kit colours make sure they don't clash. After that is a screen where they can change settings and team management like their line up, and other details which to be honest they probably won't care about, it auto selects play so they can just click. One thing you may want to check is the game time the default I think is 6 minutes a half, you may want to put that to 4-5, it can go up to 20 so that's a 40+ minute game total. You'll then have a loading screen which will be a training drill, once the game has loaded ( around 15-20 seconds ) you'll have to press a button to start the match. Now they just play, unless there's a bad injury and they have to make a substitution which you select the player that has a bandaid or ambulance by their name and then go down the page and there will be subs and choose one. This screen will come up for a red card also. If they score people do get bored of replays they show each goal 3 times just press a to skip them. Apart from that the only time play will be interrupted is half time and full time.
Career mode - choose to be a player or manager. A manager give you more options you select your team and play every game in the season, you have targets to meet or you get fired such as win the league. You can also do transfers during the transfer window, will have to deal with players who aren't playing and are upset and may want to leave and injuries. You play out multiple seasons. As a player you can be a player already in the game or yourself in which they will have to make a character, then they train, get sent out on loan or sold, play the game but you have the play as team option where you play normally, or play as player and you just control yourself, you may not play every game or the whole game and the game finishes when you're subbed off. You still have targets but instead of win the league it will be "have a rating of 8.0+, score 3 goals" etc for the next so many games. If they play as the player then you might want to change the camera angle as it'll be different, just go into settings and camera and make sure all are set to TV.
Journey mode - this is new it's like player career mode but it's a cinematic storyline where they choose how to answer questions and behave and it impacts them in the game. They'll have training which will also dictate whether they start or will be benched. The storyline is meant to mimic a real footballers career it follows Alex Hunter who gets one last chance with his friend, signs a contract at the the same club as his friend ( you choose it's automatic), then he gets sent out on loan whilst his friend is kept and they become rivals and it's how you make your way up. There will be highs and lows so if they play they may get disheartened but shouldn't.
Skill games- this is where they practice little mini games on different aspects of the game: passing, shooting, dribbling etc they earn a score and can progress to harder challenges. It has a leaderboard so they could have a friendly little rivalry.
Practice arena - on the same option it says skill games there's an option to turn right on the right stick and it should give you practice arena the default here is 1-1 with the keeper and they can just practice shooting or if they press the button on the left middle it can change to a certain number of players on the pitch, taking free kicks corners etc.
Online multiplayer - this you will need an Xbox live subscription for they start in division 10 you can choose any team to play with and the game tries to match you up with a player of the same * team and skill level, they win a certain number of points (1 for a draw, 3 for a win ,0 for a lose ) They will get promoted each season is 10 games.then if they lose enough they may get relegated again
FUT - this is their biggest mode it's like a card game, you collect players and then you can use them in game.this is both online and offline, but you get players in packs from coins you get from selling players you already have on the transfer market, or playing games. You can buy them with real money ( largest bundle is £79 worth of ea points the draft and can be bought with 2 clicks that's why I said be careful about your card) it works the same as online multiplayer but you can only use the cards you have, and each have to have contracts and fitness to play, which you get by applying cards you get in packs or the transfer market. Chemistry is also important to how the players perform and you get that by hosing players of the same nationality, club, or league)
Draft mode in FUT is where you enter and you open random packs choose one player from each pack and make a team, this gives you an oppprtunity to play with the best players for up to 4 games but if you lose then you lose them players. You get prizes which differ depending on if you win 1,2,3 or 4 games.
Weekend league, each day you can enter a tournament up to 4 times to get into a weekend tournament where you can win rare prizes such as exclusive packs with better players.
Hope I've covered enough! Sorry if some doesn't make sense my iPad goes funny if I write too big comments for some reason!