They cannot, without reasonable cause, prevent the parents of a student from attending performances. Yes they can decline to teach the Ingham children but believe me if they went to the lengths of notifying Creepy that his children could remain in classes but he was banned from the premises because they suspect he's a kiddie fiddler he would have a very strong case for defamation. Since, as others before me have pointed out, he's never been charged with any such offence, much less convicted of one.
So the only solution for that school is to decline to accept the girls back. Once they're gone, their disgusting father is no longer a problem to the school. Though they will probably still need to deal with the adverse local gossip about how he was let in to begin with.
I don't like those kids, especially not Esme but I reserve the right to feel sad that yet one more chance for them to socialise in a normal manner has been ripped away from them by the thoughtless, selfish, stupid actions of the pair of imbeciles raising them.