The general theme might be about strange and humorous body - related situations considering the title. It could be about things that are a bit out of the ordinary and make you laugh.
Since I'm not entirely familiar with the content of 'i need a new bum and other stories', I can't give a definite recommendation of a particular story. But generally, books with such quirky titles often have stories that are full of unexpected twists. The stories could be about things that kids (or even adults who enjoy light - hearted reads) find funny and interesting, like the idea of needing a new body part in a very fictional and comical way. It might also have stories about characters getting into all sorts of zany situations that are related to the body or other everyday things but with a very imaginative spin.
It's hard to say for sure. Some stories about beach bums might draw inspiration from real people or events, but they might also be completely fabricated to capture the reader's interest. It really varies from one story to another.