Like most 4 year olds (I hope anyway!), O's three favourite words at the moment are poo, wee and bum. It is therefore no surprise that 'Cinderella's Bum' by Nicholas Allan (also responsible for the excellent 'The Queen's Knickers') is one of her favourite books at the moment. It's very funny - I guarantee you will love it too!
The opening line is : "My big sister's always worried about her bum. But I think it is lovely!"