I take TB's view on this one ... the MPAA rates the movies, and it's up to the theaters to see to it that the ratings are adhered to, (which they do an extremely poor job of, in my opinion, urged on by ticket sales and those horrendously inflated concession prices)... if it's not the responsibility of the ticket person/theater employee to make sure children are not admitted to R-rated movies, whose is it?
The parent/guardian on one end, yes, but ultimately it's the people showing the film that enforce the ratings. If not, then any kid can walk in off the street to see a movie of any rating, and if that's the case then there's no need for ratings in the first place. Also, if I were the movie theater, I would refund the ticket price, regardless of who's right or wrong ... that's just good business.