Sylvester and the Magic Pebble by William Steig

 

The author of Sylvester and the Magic Pebble was the creator of Shrek.  This explains the humour and the light touch of the story.

 

Sylvester, a young donkey, finds a round red pebble that he discovers has magic powers.  He learns about the downsides of wishing for what you want but, don't worry, it all works out in the end.