She is obsessed with talking about poo and farts at the minute and now keeps saying bum. But today she said 'I ate a bum at school today' and was laughing. I said 'A bun? What bun was it?' She said 'No, a bum, bum' I asked her what she meant and she said she was joking. Explained not to joke about things like that. She said someone in her class has been saying it. Should I be concerned?