As an instructor we have to appeal to the parents wishes, but we also want the children to come because they want to, not just because they are being forced to be there by their parents. Children who are forced to come to the class and really don't want to be there, can become a distraction to the students who want to be there. Children, on the other hand, have their own reasons for wanting to join the martial arts. The younger ones want to be like their favorite TV super hero. These are big advertising and marketing strategies that pull children in.
Older children are more concerned with learning self defense.
Children, and some adults, think there is magic in learning the martial arts. Some children quit because its more work than they anticipated. To keep children involved you have to give them a reason to stay. One aspect that keeps kids coming back is the goal of achieving rank. No matter how the argument goes, rank is an important part of training, especially to children.