I really like Patty K's answer! lol A propos of drugs, I have heard of parents occasionally giving their kids a dose of Benadryl, even if the allergies weren't acting up, on the belief that it would help the kiddos sleep better. something to consider....
Definitely bring snacks. Drinks too if you can get them on the plane with the rules about liquids.
Everything you are bringing sounds good. I would think about adding some travel games e.g. Travel Checkers (or chess if they play), Travel Scrabble etc. These are the same as the normal versions but have some way to keep the pieces from being jostled out of place (e.g. the checkers may have pegs to keep them on the board; our Travel Scrabble is plastic squares with letters on them , these stick to the plastic game board). Even if they don't play the game now, you could teach it to them, or they could just goof around with it (make patterns of red and black checkers, make words with the Scrabble letters but not in cross-word formation etc).
A deck of cards or two would be good too. If you know any dice games, bring those along. My one son loved origami for awhile. If we had been flying with him I would have brought that stuff.
How about a journal for them to write/draw in?
Have fun!
K. Z.