DD9 has always been pretty interested in philosophical/nature of existence issues and has really been on a kick with it recently--how do we know what the true nature of reality is, why am I me and not someone else, where would I be if I had never been born, etc.

Any recommendations for kids' philosophy books that are fun and interesting and speak to some of this? She's MG, probably topping out around a 7th-9th grade reading level, so nothing adult yet.