Jon really was in pain and crying for real, but he also knew that for a little girl to cry would get a lot more sympathy than a teenage boy. "Mom... he hit me with my own rock! I tried to take it back and he hit me!"
"There, there, Jenny," said Jenny's mother, and handed her the rock.
Jenny quickly whispered a wish. "I wish I was unhurt, alone in my room, and I wish Mikey forgot about the rock."
Once Jenny disappeared from the car and reappeared in her newly pink room, she thought a bit and made another wish. It might be possible for Joan to become her correct age. Mikey had wished that he and Jon trade ages. He can't undo it--but exactly what was "undoing"?
"I wish that Mikey and I still traded ages, but that now, after the trade, we once again become the ages we were yesterday." That should do it. The trade still happened--he and Mikey just changed a second time, after the trade.
Jon realized, as his body grew, that he was changing. But he was still a girl and couldn't be much taller than 5'2"
"I wish I knew why I'm like this."
The voice rang in his head. "The wish you would be a little girl is still in effect. In order for you to be a little girl and still be your correct age is to be a short girl."
"Great" Jen said