It's Complicated? It's possible that if a teen commits an offence heinous enough to be tried as an adult, they'd also be sent to an adult prison. There won't be under-twelves in a prison of any sort, since they can't be held criminally responsible for their actions under Canadian law. Teens guilty of sex crimes certainly do get thrown in with other teen criminals (and often aren't adequately monitored and reoffend while they're on the inside).

But in this case, I think whatever idiot posted the message meant "there might be kids in houses next door to the prison". Which begs the question: how many prisons and jails are actually in residential areas? I'd bet it's uncommon, but not unknown.