If I'd witnessed such behaviour, before choosing to do nothing I would ask myself: Which worst-case outcome can I accept with least regret...

Possible bad outcomes of inaction:
...The girl has a baby to put up for adoption in a few months' time?
...The girl gets an STD or post-abortion infection that renders her infertile?
...Their bare butt antics end up on youtube?
Possible bad outcomes of action:
...The parent(s) or the kid(s) screams at me or worse, and calls me a liar and tells all the other neighbours I'm a pervert?

In any case one need not rat. Why not speak to those who were careless? If you decide to talk to the girl alone, take a female witness, ideally someone closer to her age than yours. If you saw your neighbour's 5 year old & friend run out from between parked cars would you do nothing? Consider that teenagers are adult bodies often accompanied by a child's underdeveloped impulse control.

Rightly or wrongly I would choose--not gladly but I'd still choose--to risk making an enemy over the risk of life-long damage to a teenaged girl any day of the week.