While banning all types of child performance would not be reasonable -- think of any movie that had NO children in it, I had a difficult time trying to think of one. The impact on that market would be fairly large. However, if there was a ban on things like pageants, I think the impact would be pretty heavy as well, in a positive way. I think things like pageants oversexualize innocent children.
Some activities, such as physically straining performances, children should not be allowed to participate in. The reality is children are not ready for all activities, for obvious reasons, and so sensible limits need to be taken to protect them, and banning them from some activities unfortunately is simply necessary. They'll understand when they're older.