I am pretty sure that once they are two they have to have their own seat.
As for what to take - lots of snacks

Especially lolly pops ( great for ears on the way down) things in little packets (even of you are a mean mummy like me and don't usually let your kids have them

)
Some colouring books and pencils (cheap ones as you might misplace a few) or sticker books. Books to read. If you have a smart phone or tablet - get some kid friendly apps - totally saved us when we took ours to Fiji aged 4 1/2 and nearly 3.