Read all the small print for flying!! Check exactly what baggage you can take, how and when you need to check in (e.g. in advance of arriving to airport, do you need to print boarding passes in advance?) make sure your liquids are correctly packed, and electronic device easily accessible in case you need to get them out to be scanned. These sort of things tend to catch people out and I imagine hard to deal with with kids in tow! I usually also take some essentials like oil, salt, pepper, washing up liquid, washing tablets for clothes etc as I hate buying it all new from supermarket and leaving behind. Depends on whether it's worth your suitcase space or not!
I would say take at least 2 sets of swimming stuff, one to wear whilst the other is drying. Realistically you only need to rinse out at the end of each day.
Take lots of suncream!!! And insect spray.
Make sure you have a little first aid kit packed, savlon, plasters, tablets for adults and kids, etc
Depending on when you arrive take something easy to cook for the first night (bag of pasta and jar of pesto) and a small carton of uht milk and cereals to pop in your suitcase for first thing. This has saved us many times when we've got to unpack and sort kids and work out the new place... when going to a restaurant or the supermarket is just not going to work!
Games for the pool e.g. a blow up ball, table tennis set?