Now, my understanding was that something had to be fixed in a tangible form. It's not at all clear to me that the appearance of one's child on stage is protected by copyright.
Any official video tapes or photos would be protected by copyright. But when I take a photo of a tree, it's not the tree that is copyrighted. It's my photo. Copyright does not prohibit somebody else from taking a picture of the tree.
In this instance it may be that the choreography is the creative work protected by copyright. Maybe costumes as well. Still, I think fair use ought to extend toward snapping a (non-flash) picture of your kid on stage, instead of shelling out $$ for an official photo.