we have travelled quite a lot with dd1 nad dd2 who are dairy and gluten allergic.
you cannot rely on the airline to cater - they do not have useful food categories, imo, and some of the meal choices are, quite frankly, unintelligible.
book as early as possible, but do not be surprised if what you order either doesn't turn up or is not actually suitable.
take as much as possible with you, as well as a letter from doc stating why you need to take these things on board.
i have managed to travel (to US) taking along far more liquids than normally allowed, due to dd1's ASD (she wuold only drink one type of juice at that point, from a 200ml carton ) and also taking along "wet" foods (airline guidelines usually say take along dry foods, eg sandwiches/snack type foods)
lakeland do an excellent thermos which is widenecked, and keeps food reasonably warm for up to 7 hours. they have saved many a situation for us.
I usually use dd1's handbaggage allowance to pack food for both girls, and dd2's allowance to pack spareclothes/toys etc for both. trunki suitcases are a good size to pack lots of food into, and are also a welcome distraction for children at airprts (and on holiday too!)
Before now, I have taken along for the girls in a thermos:
fish pie
sausages/chicken nuggets
curry
spag bol
as well as the more usual snacks: raisins/dried fruit bars/crackers/fruit
we found the US to be really good at listing what was in various meals, and reasonably good at catering for allergies (Australia was better, which was a surprise to me). we were self catering, which I find easier than stressing abut allergies, but when we did eat out staff were very helpful