The kid's definition of jail

Yeah you heard me right "JAIL". The word came out of my 3 (almost 4) year old daughter's mouth last night.

"Jail?" I asked.
"Uh-huh jail," she answered.
"Where did you hear that word?" I asked.
"I dunno," she said with a shrug.
"What do you think that word means?" I asked.
"It's where they take the bad guys and lock them up in a cage!" she said, and was quite animated about it.

Oh boy. I think some kid at school has been watching age inappropriate material. But she pretty much has the definition down to a T. Not exactly how Webster's would define it, but it pretty much sums up what a jail is.

Is it a bad thing for a little kid to know what a jail is already? I would rather she not know YET, but maybe that's just me being a protective mom and trying to preserve her innocence.