I don't know, children do have anuses, and they are known for their curiosity. I'm sure many a parent has had to dig lego bricks out of various orifices.
Children have a few things that depicting or discussing would result in being blocked from most schools, even when stopping well short of pornographic depictions.
"You can lead a horse to water but you can't stop it from sticking Lego up its bum."
Probably my favourite quote from the Inbetweeners.