Inspired by the thread about being worried about first holiday with a toddler.
Mine are past that age but from what I can remember:
Lots and lots of snacks for the plane.
Feed babies on take off and landing as it helps with the ear pressure.
Fill a small paddling pool with sea water on the beach so DC can play in the water in the shade.
Portable black out blinds (if their room is dark at home May give you an extra hours sleep in the morning!