sorry this is long but something horrid happened earlier which i need to get off my chest
was at playbarn with my two and my friend's dh (call him D) and his dd. i laughed a couple of times at an older girl (5ish) pulling felix around onto equipment and stuff - he looked happy, no problem.
I then looked up again to check where he was and saw the girl lying back on the top, flat end of a cylindrical piece of softplay equipment in the ballpit, legs open and kind of dragging felix up and down on top of her. My usual response with children would be to think it was just rough and tumble but there was something not right about it. I said to D, i don't feel happy about that, I'm going over. By the time i'd got over there, she was in almost the same position but with her legs hanging over the side and was putting F's face between her legs.
Even then, it could have been innocent but I just didn't feel like it was, so decided to take F back to the table with me and just said something like 'Hello. Can you come and sit with Mummy now please' (not a cross tone or anything). Cue the girl trying to physically stop me taking him by pulling him back by the arm as i was trying to lead him away with his other one. 
i said something like 'He's got to come with Mummy now. Where's your mummy or daddy?' She said 'My daddy's not here. Is your daddy?', I said 'No, he's at work.' She said 'My daddy's at the pub'.
I kept an eye on Felix and he wasn't near her again after that.
A bit later, we were sitting in the baby area and she was playing nearby with some other children and kind of fell backwards (not badly) and then put her hand between her legs and kept it there. After a minute or so she said "I hurt my nonny.' and then leant over to me and said it again.
With all of these things put together, I felt like something was going on with the girl, in terms of abuse 
i can't stop thinking about it and feel liked i should have done something. dh says it wasn't the kind of situation where i could have done/said anything and hoped for a positive result.
poor little mite.