Hi M. - my son is dairy sensitive, so this may not be totally applicable. He is still nursing (at 17 months), so both he and I do not eat any dairy. But, we were just visiting my inlaws in Germany and he and I both ate some sausage that turned out to have milk as a major ingredient. That night, my son was totally bonkers. Imagine the most revved up energizer bunny ever running around at 3am. This is totally unlike him, so I looked up the sausage (gelbwurst) ingredients on the web. My point is that dairy can have major impacts on sleep and behavior, depending on the child.
Also, my son drinks rice milk at day care, but I'm not sure what I would do if I weren't still nursing him. I haven't tried soy because when he was an infant he reacted to me drinking soy milk.
Good luck!
C.