If you have a package then you will be told when you need to be ready for the bus. If the flight is at 19:00 you may be leaving the hotel as early as 14:00 and spending quite a bit of time at the airport, because the bus might also be picking up people with 17:00 flights. Not much you can do about that if you want to stay until 16:30 on the last day, unless you take a taxi to the airport.
The hotel will provide luggage storage. Some places also provide a day shower room for guests who are leaving late. You will need to leave your room by 12:00 and that might be 11:00 or 10:00.
Get a GHIC for yourself and everyone else who is travelling (one per person). This will cover you for medical expenses in a public health centre or hospital. It doesn't cover repatriation though, so make sure that is in your travel insurance. But if you have an option to add or omit actual medical treatment itself in the insurance, the GHIC lets you omit that.
In Spain the medical system is a lot like the UK: Their NHS equivalent delivers care free at the point of delivery (just flash the GHIC) and there is a substantial BUPA-style private sector. If you need an ambulance or a doctor and ask your hotel, they will probably call something private as they often get commission.😠So before you travel, find out where the nearest public medical centre is, and remember that the EU-wide emergency number is 112.
Check with your phone provider what you can do with roaming. Many UK operators have stopped providing like-at-home roaming to EU destinations, but some offer a package where you can roam for £2 per day or similar. Or if you will mostly be by the pool at the hotel and don't need to be online the whole time, you could just use the hotel WiFi and WhatsApp for calls.
You can pay by card almost everywhere. Your UK bank will charge you about 2.5% on top of the official interbank exchange rate for this. You can reduce that by getting an account with Wise or Revolut, where you only pay about 0.5%. But you have to keep the balance topped up on that. Up to you if you think it's worth it. It's useful to have an extra card, of course. If you are presented with a card machine that detects that your card is from the UK and says "Do you want to pay in pounds or Euros", always choose Euros as the transaction in pounds will cost way more than 2.5%.
Don't take too much cash. Order €200 from your bank before you leave, or change pounds to Euros at your local pawn shop (like Cash Convertors), which give surprisingly good rates. Money changers in the resorts can also have good rates. Do not use exchange offices or cash machines at the airport at either end, as the rates are eye-wateringly bad. When exchanging money (anywhere), always ask "If I give you £100, how many Euros will I walk out with", as there may be hidden "service charges" that are not mentioned on the rate board.
You can also get Euros out of a cashpoint in the resort. Use a machine attached to a bank rather than the Euronet ones that are freestanding in the street. The machine will charge you about €2.50 for the transaction, so take out €50 or €100, rather than €20 per day.
Do not take more stuff than you really need with you when you leave your room. You don't need to take your passport out with you. Officially you do, but you will never be asked for it unless you hire a car, and it's silly to lose that if you get your bag snatched when all the thieves want is cash. Take €50 and one card. If DC is reliable, get them to carry €20 and the other card in a zipped-up pocket of their shorts. Leave all of the other cards that you don't need (driving licence, NHS card, Costa loyalty card) in your room at all times. There will probably be a little safe, but don't worry about that too much - any thief will be able to open it, and they are only interested in cash anyway.
When sitting in a café, raise the leg of your chair and put it through the loop of the strap of your bag, so it can't be yanked away. Also consider getting a rucksack that has a zip-up pouch on the same side as the straps, so that it's flat against your back when walking. (I'm trying not to worry you about theft - the chances that you will be a victim of crime are pretty low - but a few people get robbed every year, and the idea is it for not to be you, or if it is, to minimise the damage.)
Make it into a bit of an adventure for DC. Get them to learn a few Spanish words, not so much to speak as to be able to read on signs and labels: Departures, Baggage Claim, ice cream (and a few flavours), etc. Our kids always felt like they had access to a secret code when something they had read actually showed up and they understood it.