They expected a 10 year old little boy to handle shame and humiliation, so why shouldn't they be able to handle it just as well. Normal parents would never expect a child to cope or handle something, they themselves couldn't. Perhaps now they know just how it feels.
You did what you thought was right.
But did any of the part where it doesn't help the kid sink in?
Like I said, I wouldn't have known who it was and the knowledge that the son of X and Y wet the bed at 10 years old.
Thankfully I don't even know the people.
But in order to know that the parents are bad, it necessarily gives more people knowledge of the kids' situation than even him holding the sign.
It outs his situation all the more.
But what the hell? If that's out, why should name and address be sacred?
You are helping hold that sign - and are keeping that sign out longer than the kid held the moment you named parents and address.
Your motive was to draw attention to the parents. But the unintended result is letting more people know who the kid is.
Good work! By outing the parents, you are in solidarity with their motives.