Just a caution about letting children have milk on a plane: ONLY DO THIS IF YOU *KNOW* THAT THE CHILD DOES NOT GET AIRSICK!!!
Milk does not cause airsickness, but if a child does feel nauseous, the presence of dairy in his stomach will almost guarantee that he will end up throwing up. This goes for any dairy, including yogurt, cheese and cheese-flavoured things, like Goldfish crackers.
Small children cannot hit the target using an airsickness bag; trust me on this. Also, they almost never manage to give any warning if they get airsick; and the mess will be VERY unpleasant if there is milk involved. Remember that if they feel ill, they will almost invariably turn toward Mommy, so Mommy will end up covered in the yucky stuff.
If you must give a child milk whilst in transit, be sure to carry not only a full spare outfit for the child, a large ziploc bag and an entire container of wipes, but also a spare outfit for yourself, including underwear. You might need it.
If your child has a weak stomach or a problem with airsickness, it's best to keep a small pail or a large plastic cup handy on flights.
We used to always carry a small beach pail, as it had a handle. If this is an issue, keep the pail on the seat next to you at all times; don't put it under the seat in front of you or in the overhead; there will not be enough time to grab it if your child gets ill.