Oh I feel for you :(
We had a similar worrying situation with DD over the behaviour of a boy in her class, also sticking tongues in mouths & some of the other stuff she came home with that he was doing - even worse, though thankfully as it was in a School situation, from a sexual point of view we at least knew it couldn't get too out of hand in what he was doing with other kids - if that makes sense, though he was also a bully, but thats another story as such - thankfully he is no longer in her School
I'd keep an eye on her, see what she comes out with further, watch her mood etc & it might be a good idea to have a word with her Teacher too, just incase, & if worst comes to worst (which I doubt from what you've written) they will have the back up of a counselling service who can help her with it all
If its any reassurance my DD was older than yours @ 6 when it all started, she did end up seeing the School counsellor, but that was also because of the aggression she'd witnessed from the boy towards staff, being bullied by him as well as things like being pinned by her throat & having his tongue forced into her mouth :(
A while down the line & she is fine, remembers the boy as a bully & smashing the class room up etc, but has dealt with it all really well & remembers little else - if anything in a weird way its done her good in that she's very mature in how she deals with conflict
it is all a bit "playing Drs & Nurses" which all kids do to a degree...(eek), but with him being so much older its more of a worry & as such I would keep your DD away from the older boy though, at least until you feel sure he IS okay, I'm a firm believer in trusting your instincts
Good Luck