This was a single family home, 3 bed 1 bath. Incidentally, the basement was smaller than the house itself, which I guess why the hatch wasn't in the bathroom itself(which was over empty crawlspace).

I'd never considered the child aspect, but that may explain why it had a lock on it(the turn up, push/pull, turn down kind, like on public restroom doors, unsure what they're called). Also, why it was so heavy. I wish I'd taken pics of it. From memory, it was probably 8"x8", about 2" thick. A small child could fit in it, but would probably have difficulty opening it.