In the world of literature, isn't it more commendable for an author to write about topics they truly believe in, such as in the case of HRH Princess Michael of Kent - conservation, endangered species, kings and queens in history etc. I believe she also did lectures on the subjects. To me it shows far greater maturity than a celebrity writing a few pages of drivel in a children's book.
Don't get me wrong, I'm not saying that writing a children's book is a simple task, as JK Rowling admits. It just seems that several celebrities choose this easier option, which demands far less literary skill, just to make a fast buck or to further their image.