Dream to be honest me too! I was waiting on someone else saying, yes I did that too as I was starting to wonder if my childhood was a bit weird haha. I very clearly remember being shocked as I thought we all looked the same under our clothes!
I do think there are degrees. I know of one incident that involved vaginal penetration by object - I witnessed unfortunately - and that was quite rightly followed up. Turned out the child leading it had actually been abused at a very young age.
There's also issues around consent - if it's mutual (like the situation I describe and as the other posters have mentioned) it's perhaps not so worrying, but if on the other hand it was forced in some way or part of bullying, then it's a bit more iffy, although still doesn't necessarily indicate that the child carrying it out was in some way abused.
It depends as well on what happens doesn't it, so if they're obviously imitating sexual acts and using language indicating a knowledge of sexuality beyond the norm for their age, then it's obviously worrying.
If on the other hand, it's as I experienced (very mutual, same age, and just fumbling - if that's the right word/showing/looking) then it's probably not so worrying.
However - I'm not CP trained, I don't know if anyone on this thread is, for that reason you might be best discussing it with someone who is. In any case, she does need a bit of encouragement about boundaries. Being pubescent (11, at least) she doesn't need to be stripping in front of wee ones in the street for her own sake if anything else!