When my youngest daughter was 4 years old, I was so worried about her development that I had a spoke with the owner of her pre-school. He agreed that she was having trouble focusing and was delayed, and he made a suggestion that not only changed her life for the better, but has helped everyone in our family. He looked up at me and said, "Have you ever thought about modifying her diet?" And my answer was no. I had really never thought about that. We'd been to several doctors voicing our concerns about her, but we were told there was nothing we could do until she got older (in a few years she could be evaluated for ADD/ADHD or autism).
So my first website suggestion is: www.feingold.org
It's a great starting place!