In the specific instance involving "Sweetie", sure.
Please don't tell me you think that Bob, aged 51, had never before in his life propositioned a child — a child who is in all likelihood an actual slave — to perform sex acts over the internet, until he happened innocently to be hanging out in #dirtylittlegirls, and propositioned Sweetie.
Then we should catch Bob propositioning an actual child. Surely we don't want to have a justice system based on assumptions and probabilities? Either we have hard evidence or we don't. Chatting to a virtual avatar is not hard evidence of anything in my opinion.
I don't know for certain, but I don't believe the police are making arrests based solely on an alleged perp's interactions with Sweetie. If you ask me, however, Bob's propositioning Sweetie creates sufficient Probable Cause to obtain a warrant to search his computer for evidence of other, similar, interactions, with actual children. Then he can be prosecuted for the cases where there was a victim.
Whether or not you believe a victim other than society in general (or the sovereign) ought to be required for a crime, that's not an actual requirement.
