OK. So my anecdote about spanking is invalid, but
Obviously sexual harrasment and sexual assault becomes extra disgusting when it's aimed at kids. But I will guarantee that all the scenarios in which you could traumatize a kid (and adults) with sexual behaviour is regulated in the law. The same goes for any physical violence aimed at kids (and adults), as it should be.
How? Not allow guys to jerk off in public when there are kids around, just adults? Sounds like a completely toothless law. It boils down to your definition of sexual harassment, really, and it sounds like to me (correct me if I'm wrong), that you don't think jerking off in a public space nearby to children is sexually harassing them.
You saying that you would be traumatized about watching a guy jack off is not true. There could be a case if the guy was standing in front of you, looking at you, and it would definitely be an unconformtable situation, but I doubt it would be traumatizing in the true sense of the word. But in any regard it would also be illegal for the guy jacking off (targeting someone and all that, which I wrote about earlier in this post).
"Targeting" seems nearly impossible to nail down in a legal sense. Let's say some guy is watching your kid while jacking off. You take him to court (after you give him the chance to finish, of course). How do you prove he was targeting you? He made eye contact? His eyes lingered more than some arbitrarily allowed length of time, over which he crossed the threshold from a transitory glance to a leer? All he has to say is that he wasn't targeting your kid, he was actually looking over her shoulder at the trees/walls/seesaws behind her. It's impossible to prove that his eyes were targeting a certain individual.