S.R.
I am a chiropractor, and we work with children with behavior issues all the time. While I have not used the Feingold diet personally, there are many items I suggest that parents avoid: artificial colors and flavors, artificial sweetners (nutrasweet, aspartame, splenda, equal, etc.; pretty much any product that is sugar-free), high fructose corn syrup (and corn syrup in general), and highly processed foods. I often receommend removing dairy from the diet as well.
These are steps that I take with my own children for overall health (they've never had any health or behaviorial issues). If you'd like more information, please don't hesitate to contact me at ____@____.com luck!!