M.L.
We took our daughter to a pediatric gastroenterologist and he said to give her miralax and that it's too hard to modify the diet of young and often picky eaters. Every morning since (for over 4 years) we have heated a small amount of juice, disolved 1 t of prescription miralax and had her drink it. She also does better if she has frosted mini-wheats for breakfast.
One thing the gastroenterologist said that struck home "no one should hurt when they poop" Good luck:)