Their mother really didn't like cleaning up after children, having to find a babysitter whenever she went away, and things like that. And while all parents treasure their children's childhood, that was nowhere as big a deal as it would have been considering that she had a magic stone and could even instantly have young children again whenever she wanted.
"I wish that Brittney and Kelly were 15 years old."
Kelly didn't notice this, since she didn't hear the wish. Brittney just remembered being Jon and then being transformed by a 15 year old Kelly instead of a 5 year old Kelly.
Later that day, Brittney--who still thought about playing with Kelly all the time, and who still liked only what she liked--took off all her clothes and stepped into the shower.
Instantly turning into Jon with a girly face, and still under the effect of the other wishes....
