If you want to bring water... remember, its floridated. Kids can have all sorts of things carried on. Milk, floridated water... don't know about juice. You can even buy it at the airport and put it in their sippy cups as long as you are willing to lie about it. (The guy at the airport told me to do it even! In not so many words)
My only other advice is anything can happen. Our DS was 15 months... refused to nurse for 9+ hours on the way there, refused to sleep since we tried keeping him on schedule, didn't like the toys we brought (favorite ones hidden a month prior). The carseat was a godsend. Without being able to strap him in, I would have totally lost my mind!