How common is constipation in toddlers?

My daughter use to always have problems with constipation. Prune juice worked best for her and yogurt with active cultures was always good with preventing it. How many bananas does she eat? Bananas can actually cause constipation. My daughter loved them but we had to cut the amount that she got to no more than 1/2 a banana a day...and that made a HUGE difference.