Many times I am asked this question. Here is my answer:
Lowfat or skim milk ( unless you are serving something with dairy already, such as pizza or lasagna)
Chocolate milk, if your child won't drink the plain (yes, I believe low fat or nonfat chocolate milk is ok if that is how your child will get their dairy, and if you have offered the plain and they won't drink it)
Water - Always put water out too, even if you are serving milk.
Hope this helps!