This isn’t a direct response to the question – but as a community we have to know that these possibilities exist and work on reducing the risk and space for it to happen.
I work in both schools and a shul in the UK where there is a huge emphasis on ensuring that there are no areas which are private. All offices and classrooms have windows to corridors, and those that don’t have CCTV. A lot of screening happens before recruiting. All programs, in and out of school, and shul programs, are thought through to ensure that the possibilities are reduced. Kids are taught in (some) schools about abuse (in an age appropriate way).
I can’t answer the therapy question. But again, the therapist ideally shouldn’t be working alone, and should be visable to people outside their office.
It works both ways. These measures are as much about protecting the victims as they are about protecting the potential accused from false accusations.
A lot of responsibility lies with parents. Where are your kids, are they secluded with someone, whose around during their private lesson, therapy, after school shiur etc. Be strong and resist the sleepovers. In whoever’s house.
This doesn’t answer the question of how to deal with somethingg that’s already allegedly happened, but hopefully cuts down the possibilities in the first place.