As for taking drink on the plane, the woman in front of us travelling with a child the other day was allowed the child's drink, they just had to be seen drinking a bit by the staff by the X-ray machine.
Saying that, there's usually a Boots or something once you've gone through security so you could purchase a suitable drink there which is allowed on the plane. Anything you purchase after security can be taken on board (assuming it fits in your hand luggage!).
You should be able to get fresh milk from the supermarket, might just have to get it daily and keep it in the coolest place possible. Though they usually have UHT in Spain, which children often reject, but maybe you can try them on it before you go.