Sexual abuse of very young children is unfortunately more common than one would think. They’re easy to impose upon or persuade, for one thing, and often do not have the skills to report abuse. It is likely that the teenaged girl was abused herself as a young child. She could be a family member of the little boy: that’s why I mentioned in my first post that it would have been useful to know the context. If she was a babysitter or daycare worker, there could be other victims.
I also suspect that she was abused from a very young age.
From what I’ve read on the topic, ‘abused turning into an abuser’ is one of the primary ways this perversion is passed on to the next generation.