Maybe take iPad/ DVD player along with you if you have one & feel it will entertain your DC's
Make sure you have adapters for the plugs too! Buy some snacks after check in to take on the plane- by the time you have taken off, had some snacks & toys it will be time to land again as its a nice short flight. Dummy/ drink for take off/ landing.
Layers for you all for the plane as it can be blooming chilly.
Can I ask whereabouts you are staying? Have been looking for similar holiday for our family end of May/ June
for future ref, buy your insurance when you book your holiday (not necessarily from the same supplier and never from high street travel agents). You need cover against cancellation as soon as you book.
tell your bank when you are away so your cash card will work. Same for credit cards.
keep an eye out for special offers on suncream (saw kids Ambre Solaire stuff for £3 a bottle in Sainsbury's the other day)
Lizardqueenie - we are staying in Peurto Pollensa in Aparthotel Playamar with Thomson - looks a good resort, not too tacky, and lots of facilities at hotel for kids, great reviews on trip advisor - so fingers crossed!
Remember not to pack expensive sunscreen in your hand luggage as they won't allow liquids on the plane. Sun protection t shirts or suits save masses of effort sun creaming wriggling children. You only need do faces arms and legs then.
Wherever you are there will be open water and you must never take your eyes off them. When ours were that small we used to take turns so we both got a rest. One watching and playing with DCs while the other relaxes, and you always know who is on "duty". When DS1 was 4 we arrived at our hotel and were unpacking, we were in a ground floor bungalow and he slipped out and was in the pool within seconds. Having said that ground floor is better because you don't have a balcony. DS2 used to sleepwalk and was caught rattling the (locked) balcony doors in the middle of the night. If you have a preference it's always worth e mailing the hotel to ask nicely!
I always take some Piriton, after DD1 got hayfever in Majorca when she'd never had it before at home. The pharmacist there said they don't sell antihistimines for children over the counter, we woud have to see a doctor. Luckily, a family we met had some.