There's a couple.........................kids and pedophiles!
Do you mean all pedophiles or only those who've acted on it? If you mean all, then:
1. What non-offending pedophile would voluntarily register himself and ruin his life by doing so? And why should he if he's not yet offended?
2. Offending pedophiles are already registered.
As for registering offending ones, that's like closing the barn door after the horse is out. Registering a smart phone could serve as at least somewhat of a deterrent at least in some cases.