I have had confusing sexuality and I believe that we don't choose our sexuality and that if we become something we hate then they should be given fool support. But if he is attracted to young kids he shouldn't have had one with you! I think its worth checking out that he hasn't done 'anything' to your daughter after seeking advice. If he is attracted and hasn't done anything wrong to your daughter then he deserves full support from you and profesionals, but I do understand your reaction.

When he is with your daugther he might be thinking about you and therefore gets an erection.

The main thing is that he has not done anything to your daughter - get that checked out and the best way to resolve this is to probably see a counceller.