M.K.
The sales lady THOUGHT she heard a 13 year old girl tell her 7 year old brother "Let me see your d***". I would be appalled if I heard something like that too. Now, whether it was really said is another story, but unless you come up with something that sounds plausible in place of that, I would have to side with the employee. D*** sounds nothing like we-we and, while therapy may be a bit premature (although maybe it IS needed if indeed your daughter said it, I can see how that would be disturbing), a sound talking to of how trashy the word d*** is coming out of a thirteen year old girl's mouth might be in order.
I just don't see an employee and manager tracking you down if they really and truely didn't believe what they heard.
Bottom line=three things:
1) Your lucky that the manager didn't call social services since that is what her employee thought your daughter said.
2) You'll never really know the truth but you can learn from the situation to make sure it never happens again.
3) Don't send your 13 and 7 mixed gender kids to the bathroom together anymore (were they in the same stall together or was she standing outside the stall door?).
I stand outside the door (cracked open with my foot) and yell "is everything okay?" every 10 seconds while my 7 year old is in the men's room. I've gotten weird looks..BUT.....who cares?