It is absolutely fine. Don't take too much to entertain them as you won't need most of it but make sure you are covered if the tv packs up.
Firstly check if your airline has tv's on the seat backs. If they do you are sorted as there's a cartoon channel usually. My ds was glued to this most of the time he wasn't asleep.
If they don't have them get them both a nintendo ds if they don't have one already and a couple of games each including multiplayer they can download together.
Don't rely totally on either of these though. nintendo will run out of charge at some point and on one of our flights the tv's were broken so there was nothing available to watch!
Last time we flew we didn't get given the puzzle pack they had on previous occassions. So do take a simple pencil case with crayons, pencil etc and a small colouring book and puzzle book as a surprise. Then if the airline doesn't provide it you are covered.
Try and get the window seat if you can as it's easier to ask someone else to move on the aisle seat if you/they need to get up than it is to get your child to move, especially if they are tired/asleep. I find window better than aisle as they can lean against the wall to sleep and not be disturbed by people walking past.
Take a drinks bottle (empty) and ask for extra water/juice to put in it when the drinks trolley comes round. Less likely to have spills and you'll have it for the rest of the journey without having to wait on the steward if they get thirsty.
Also take travel sweets, a packet each is usually fun. Make sure they have one on take off and landing to prevent ears popping. My ds likes the travel sweet tin type, and I let him choose before we go as we don't have them often otherwise and he's excited doing that.
Oh yes, get them a small travel bag/rucksack each so they can carry tehir stuff in it. It should fit easily under their feet/seat so they can get their stuff out of it without you having to get to the overhead lockers or carrying a huge bag yourself which you have to rummage around in. The bag I have for ds has a strap that fits over the handle of his suitcase so I don't need an extra hand to carry it if he's tired/asleep.
This isn't the case I got for ds (I don't think his available as I can't find now - it was bright blue witha monkey on it and had a matching holdall bag) but this is along similar lines, is fun and a great way to encourage them to be responsible for their own luggage too.
www.amazon.co.uk/gp/product/B002UOZ6WG?ie=UTF8&tag=kidssuitcases-21&linkCode=as2&camp=1634&creative=19450&creativeASIN=B002UOZ6WG