I was a little bit like the girl when I was younger - I am ND and probably wouldn't have been completely aware of boundaries.
At 11/12 I would definitely want to be seen as someone who was good with kids and would probably be doing it to show off to the adults and show them how grown up I was.
Honestly if I'd been in this girls situation I can imagine feeling like I had been trusted to entertain the child and it was a big responsibility.
I remember in soft plays feeling completely separate from the adults and sort of forgetting they could see us. I can imagine how she may have needed the toilet and your little girl said she did too. She might have felt it would be irresponsible to leave a little girl in there. It probably would not at all have occurred to her that the little girl might need help going to the toilet.
If it turned out the girl did need help, I would probably have panicked and not known what to do. Definitely a part of me would be worried that it wouldn't be appropriate to help her go to the toilet but another part of me might be worried that I was being silly and that's what people do when they look after kids.
At this point I would consider going to get you to ask what i should do but would be worried about leaving her in the toilet.
I imagine I would have solved it by encouraging the little girl to do it herself the best she could (by shouting through the door) and then explaining to you afterwards.
I definitely would not have actually helped her with going to the toilet as I would have known that of all the choices, that one would be the worst one to get wrong.
Perhaps she saw you seemed angry when she came back and thigh tb she was "in trouble".
It would never even have occurred to me that someone might think something nefarious was going on or that I, a 12 year old counted as a "stranger" in the "stranger danger" sense.
Obviously none of this helps you because I'm not the actual girl in this story. And of course the girl could be an abuser with ill-intentions. But it's very very unlikely. I also wouldn't assume that, if she struggles with boundaries and social norms, it is because she herself is being abused. She might just be a ND girl trying to seem grown up, doing a lot of overthinking and getting a lot of things wrong.
It's an especially difficult age when it comes to knowing expectations. Everyone is telling you you're not a little kid anymore and that you're going to big school etc. Youre starting to feel more grown up but you're obviously not at all an adult. It's a weird in between age where you often want to be more grown up than you are.
Or course be careful and don't let your guard down. Keep an closer eye next time. But I don't think you've been given any reason to believe that anything inappropriate happened.