as far as I am aware there are no chat rooms on FB, just a chat facility and the only people who pop up in that are those on your friends list
You only have to be 13 to use FB and that's where the problem lies - many, many 13 year olds use it.
Personally, I feel FB should be available to 16+ and no younger
I know all my children's passwords and I do check up on them
I set their settings up to enable only their friends to see them, not friends of friends and their posts are set only to friends