Familiarise yourself with airport security arrangements (liquids etc!).
Make sure passports are up to date, (or have an additional 6 months depending on when you're travelling).
Book seats!
Consider how much luggage you are taking, if you need to pay for additional to go in the hold.
Make sure you have travel insurance from the point of booking the holiday.
Consider ordering anything you need from Boots and collecting airside (nappies, Calpol, suncream which may be too big to take through security) which saves space and can be left on holiday.
Pack light - you don't need as much as you think you do!
Do you need your satnav? If not, does your phone plan include overseas data? Is there wireless at the resort?
Pack your rash vests/swimming aids.
Consider fast track security - usually v. cheap - if travelling early.
Work out the easiest/best car parking at the airport - might be meet & greet, might be at a hotel if staying overnight the night before?
Don't forget their favourite sleeping toys!
Kindles/ipads are brilliant for the flight - make sure you bring earphones to avoid annoying other passengers. I also buy them one of those trashy kids magazines with loads of stickers etc which I only give them on the plane (the airport is really entertaining).
Consider paying extra for speedy boarding - depending on the airline.
Don't forget chargers - for monitors, kindles, phones. I bought an EU USB plug that takes 4 cables - saving lots of space. Additionally consider adaptors.
Don't forget to check in online, add advance passenger information, when required to.
Do you need to bring your own beach towels? Those really slim/quick drying ones are good.