Their mental maturity may be a way forward, if they individually could understand that making this a practice is too much too soon for many reasons. I doubt that, by them. I think you've got a problem to face.
I don't believe that precocious sex is bad for one's health or anything essentially and objectively wrong. It's a taboo in SOME cultural contexts. Stronger in some cultures than in others, some more promiscuous, some more puritanical. In France the mainstream attitude in my generation is benevolence because our generation, and the one before, have been there. I was a late starter women-wise but my left hand has been a dependable friend since early on. My wife gave her fist BJ at 13, and she's not particularly odious as a result... just better at it perhaps.
This IS ALSO about learning. The boy is fortunate in this sense because she seems to know a thing or two. Learning late with clueless partners (or never) makes for some weird people too... From this view point, precocious sex may be better than prolonged abstinence during puberty and beyond.
This said, i also can see a legit concern and reason for keeping this, if not forbidden by law, at least repressed by parents including by our OP: If kid sex become too conspicuous in society, kids could be exposed to a growing threat from adult sexual predators. IOW, I support a social prohibition on pedophiles, and thus I don't want kid sex to become a frequent, non-transgressive act. Parents and other adults should not condone or facilitate it. They should still frown upon it, as a way to keep it rare and under the radar.* But it should also not lead to anything like public shame and police and foster homes and what not. In the case described by the OP at least, that would IMO lead to the worse possible outcome. Have mercy on them kids! They are not doing anything wrong.
* Aren't these things better when they're rare and forbidden anyway?