I am not going to write a full-on review here. I've been sick and just working up the energy to read this book has been difficult. Fermat's E. is waiting for me at the library (again) so I will probably just move right on. However, I think I understand why I did not enjoy this book as a 5th grader. I would not consider it solely a children's book. It has too much depth to it.
I agree with what you all have written already so I'm not going to re-type it. :-) I have very little to add.
I thought it was interesting that the author wrote this during WWII. I was thinking about why he wrote this and what sort of influence the war had on his writing choice.
One thought that occurred to me is that you could look at the little Prince character as a Christ archetype. One example to support that is that the Prince leads the pilot to a uniquely refreshing source of water. He is certainly a teacher.
In essence, the book is about the healing power of love which makes all things unique, and how the pain of saying goodbye is worth it if it changes how we look at the world.
And there we have it!