While carrying the single serve milks around works, I would image that would get pretty heavy depending on how much your child drinks.

I like to use the Single Serve Formula mixes for Toddlers. Enfamil sells them for sure (What I use), possibly Similac and some of the other generic brands as well. They come in these little tubes and you use one per 4oz of Milk. Just mix the powder with some water and go for it. Then you can either get some water from the stewardess or just keep a water bottle with you that you buy after security and fill it up when needed (my suggestion in case stewardesses are busy)

We usually keep some in the baby backpack even when we are home in case we need it for our day trips and outtings with the baby.

Drek