Sylvester and the Magic Pebble
Good message, potentially scary story.
I just think it's potentially upsetting that the child turns into a rock and stays that way for a long time. His parents are distraught (although I suppose it would be worse if they weren't). Clearly in this universe, magic is extremely powerful and permanent. If it was something a little less drastic, and he didn't stay that way for almost a year, this would be a good book. I really love the scientific moment, and the message of appreciating what you have. Maybe when my kids are old enough to completely understand that magic isn't real, like at age seven or eight, I might read this to them.