A difficult one, in some cases it would probably be in the best interest of the child, but in most cases I don't think it would be. It depends a lot on how bad her CP is. And how much do we actually know about what they understand, even if they can't communicate it?
Christy Brown's book My Left Foot, gives you quite an inside in to what it is like to be trapped in body that you can't control, he was probably very lucky to be in that particular family!