A boy of culture

Sexual everything being such a common thing in media combined with the fact that most children his age have access to the internet and "cute" imitation being encouraged makes it so that I'd be rather supposed if a child his age didn't know about at least one mainsteam kinky sex act

Possibly. Or it could just be that kids do weird stuff that adults think of differently and they often see as innocent. Factor in that smaller children tend to put their mouths on things more and use the mouth more to explore the world. The. There’s breast feeding. Theories abound that humans beneficial and somewhat rare face to face love making is related to the somewhat rare prominence of female human breasts, and the general size fixation around them. A big question being- did human males start hitting it from the front because breasts looked like asses, or did breasts that look like asses develop through evolution to attract males to face to face intercourse? Either way- a well endowed chest pressed together looks very much like a butt with nipples. So it’s very possible a child might try to suckle a butt due to the resemblance. Or- he’s a kid and they do weird shit. Whichever best suits the gallery is the theory I propose.