Your friend tells you they're a paedophile

[robvalue]

I understand the desire to warn the family.

As a thought experiment, let's think it through. How would it play out?

You're Johnny's friend, and he confides in you he's a paedophile, as per the OP. He has kids. You're as certain as you can be that he hasn't harmed them, and doesn't intend to. (If you're personally unable to ever believe such a thing, then this post won't apply to you.) When he's out, you go see his wife and tell her. We'll assume she doesn't know already. There seems to be three likely scenarios:

(1) She believes you, and tries to leave him, taking the kids away with her.
(2) She believes you, but sticks by him and tries to get him help, while taking measures to safeguard the kids.
(3) She doesn't believe you, but it may leave a seed of doubt which could affect their relationship.

Whatever happens in this situation, your friendship with Johnny is probably over, he feels betrayed, and has one less really good friend for support. Perhaps he wanted to build up to telling her himself; perhaps not. Maybe if he did plan to, it would make you more inclined to support him in secret until then?

It's all going to vary wildly depending on the situation of course. There is no wrong or right way to do things. But I wonder what is likely to happen, and whether things have been made better or worse overall for having gone to his wife. This is just an invitation for discussion, to think through the reality of the situation.