I'll share some recipes that will help. Yes, recipes. First... a big concern here is regularity -- believe it or not. If your child isn't regular your chances of training success significantly DECREASE. Given most diets today kids do not get enough fiber to remain regular. I know it sounds scary -- to have your child "going" more but trust me.
"Going" regularly is critical as is incorporating “whole” (dark green leafy vegetables, fruits, nuts) into your child’s daily diet. I know, I know "picky eaters" but I've heard it before. I found an answer that is fast and easy (on YOU!) to overcome picky eaters too.
No, it's not sneaking foods in (that seems to fail too) but presenting the fruits and veggies in a irresistible way to your child. Honestly tempting. Here's one recipe http://budurl.com/ncpj and here's another involving chocolate http://budurl.com/bw3m (raw cacao sweetened naturally with dates) that your child will BEG you for (and you can serve as much as you want!).
Of course, these can also be used by mommy to shag the extra baggage and I've had a few friends discover that as well!