This 19 year old may not be a pedophile in the usual sense of the word, but he does have a young man’s testosterone levels, probably has sexual urges he doesn’t understand.
His parents do need to watch him closely, need to stop taking him to children’s playgrounds, and explain to him the wrongness and the consequences of touching children, to whatever degree he understands.
And that is a rough rough job...how do you watch your child and still work to provide for him when you are gone? It’s not like the state is full of safe healthy homes for the mentally handicapped.
I will add though that he probably does not understand the consequences of touching someone as many do have impulse control issues. As a parent who has been doing this for 19 years I am tired, it is wearing more than you know and for a moment I may drop my guard which I think happened in this instance, and I am not saying it was right, just stating that it does happen.