How To Overcome The Problem Of Enormous Feces That Hurt While Passing Them ?

My daughter has 4 children. They eat a well balanced diet with very little processed food. They all suffer from the same problem. They have enormous faeces which hurt when they pass them. They are only young children so this inevitably leads to them not wanting to go to the toilet which then causes an even bigger faeces. We have looked at their diet and added extra bran etc. but this has not altered the size. We can only think that their bowels must be larger than their rectums, is this possible. The only other thing is that they are of mixed race, their dad is from Fiji their mother is British, can this mixture, or any mixture, lead to physiological incompatBilities