One other point I want to make clear is that the only reason I said anything about asking a sheilah in the first place was not because I had a “hava amina” that the halacha might be that you don’t have to protect people from molesters.
The reason for asking a sheilah was to determine whether or not doing x in situation y is really protecting people from molesters.
Many times people make incorrect assumptions about these things. You gave an example that seems to be pretty clear-cut and is less likely to need a sheilah. But not every case is so clear-cut. And even in the case you gave, the question is still – “what is the best thing to say to who?”
I actually do not understand why you think that telling your relatives about it is the right approach. Why wouldn’t you tell the Rav of the shul and/or the Rav of the community and/or the police so that they can get him out of the shul and community and preferably in jail??!!
When my friend saw someone in her shul acting inappropriately with a kid, she told someone official. I forgot who she told – maybe it was the Rav of the Shul or someone else official – it’s a completely Frum city, so all the official people are Frum. They blacklisted him from the Shul and basically kicked him out of town, and his wife divorced him (my friend felt bad about that. I don’t know why).
That is how you handle things like that, and that is much more effective than telling an individual who goes to the shul and can’t do anything about it or than posting rumors online.