Do not say anything to anyone. Should not even have mentioned it here, I think.

You just don't know what the reaction to your best intention will be.

Even a simple "You have been seen, please be careful in every way" might instigate a nasty backlash against you. Accusing you of being a voyeur and saying inappropriate things to the girl.

Leave it, forget it.

They are kids doing what kids do. All perfectly normal. At least they weren't doing drugs or making a ritual sacrifice.