Really surprised by some of these responses as I don't think I'm a prude or overly dramatic at all but I would not be ok with this. When I was 11, we had boys sneak into a sleepover. There was french kissing and seven minutes of heaven (entirely based on some john hughes movie, nobody really knew what they were meant to do), and a certain element of 'I'll show me yours..etc'. It depends on the kids but I wouldn't rule out something like that - and again, I obviously wasn't traumatised for life, but some children are more sensitive than others and wouldn't react well if there was inappropriate joking, etc, and suddenly they were in a situation they didn't fully understand.
A girl in my school had a boyfriend at 9, everyone thought, 'oh, how cute' except she was pregnant by him at 13. At 9 they were doing more than holding hands - not much more, but they were experimenting with what they thought boyfriends and girlfriends should do based on big sisters/brothers and then years later it escalated.
I am very pro not sexualising children, but in a culture that does that for you I think it makes sense to be realistic about the actual risks they face. And also: all this was a very 'naice' middle class school and area. I see a lot of people on here who say 'my child is a very young x' with the subtext they go to a nice school and won't have been exposed to these things but I can say for certain our parents would have said/thought the same. Totally don't mean to scaremonger, and 90% sure it wouldn't be a problem in this case, but ultimately: at 9 at least some of the girls will likely have started puberty to some extent. Won't it be a bit embarrassing? Will they all change separately?
I would have thought it might highlight differences that shouldn't be there when they're playing, and if its allowed now when do you stop allowing it again IYSWIM. When they're 13? 14? 12? Isn't that signalling you suddenly do expect them to be sexual? I would view this as a giant can of worms and go with picking her up at 9.