“Nah, it’s those fountain statues that pee continuously that are the worst influence!”
By the way, you’re right. I do not know how to influence lawn statues to children, but in that flower bed, in which urinated my nephew, the fountain has been installed: a boy holding a ewer (it was not working). Apparently, my nephew decided to help this boy 
However, I was more concerned with the question of how to wean him from peeing down, standing on a tree.
He refers to it as a game. He says that he does not hesitate, because this is natural and fun. In truth, he is very innocent in their arguments. I’m worried not so much an ethical dimension, but his health.