DD is 4 and has made a new friend a few months younger. Everytime they're together they want to play mummies and babies which seems to consist of her lying on her back with her legs in the air while friend pretends to wipe her bottom and change her nappy. Yesterday her friend had lifted up her top so DD could have breast milk. I've tried to distract them with other games etc rather than make a fuss over it but am now getting fed up with it. Any ideas?