Say, not bad at all! I could actually see something very much like this being made in an early 90's pre-school children's show. That's a compliment, I promise:} The rhyme scheme was solid and flowed pretty well. I did notice that you happened to change one particular aspect of the story in that Jack is usually depicted as selling his family's cow instead of a mule. Was that for the purpose of the rhyme scheme or just to give the tale your own personal touch?
I know your pain. I recently lost my grandma to Cancer and the wound is still fresh. It's only my family's love and support that has kept me going. That being said, never lose hope that your grandpa's situation may improve. I wish you and your family all the best.