polyp in 9 year old...help

My daughter who is 9 has always had problems with constipation. 2 years ago she has bloody stools and the Doctors kept telling me to feed her more fiber (which she already eats). I took her to Mayo for colonoscopy and they removed a large polyp which was non cancerous. However, I am worried they will return. Is there something for this age child to do that is safe to keep this from happening again??? I think she carries alot of weight because of these dsigestive problems too. She weights 127 and is 4ft 6 inches. Any help would be appreciated.
Thank-you