This world was a lot like their home. Perhaps it was their home world, they hadn't been able to tell yet. They had been born to the same mother and father, and though they kept the same hair and eye color, their mother was very pretty, and they looked to have inherited her looks. They were both four years old now, and everyone marveled at their intelligence.
Naturally, Nina and Kimberly hadn't bothered to explain their origins. When the got older, they'd try to build the transmitter and see if they could bring this Earth and the previous one they had visited into contact. They were playing in the park one day, when Nina found something shiny, a gold ring.
Bringing it to her sister/daughter/friend (there really were no words to describe their relationship, which began long before they had even been born), they decided to keep it. They would pass it back and forth between them.
One night, Kimberly had the ring, clenched in a tiny fist. 'It's not bad being a child, I have Nina, and great parents. But I miss being a woman sometimes.'
"I wish I was at least 16 again." Suddenly, she began to grow rapidly, her nightgown ripping as she outgrew it. Her legs and arms grew longer, her hips widened, soft wisps of golden pubic hair erupted from her crotch. Her hair grew longer and fuller, and she began to grow breasts.
Nina woke up at this, and stared in awe at her 'sister', who was now a girl of sixteen. "But how?"
"I don't know, I thought about being older, and then I was!"
Nina tried to see if she could duplicate the feat, but she could not.
"I know this sounds weird, but maybe...here." She handed Nina the ring.
"You can't be serious." But Nina clenched the ring. "I wish I was seventeen!" She began to grow as well. Kim helped her get her clothes off before she ripped right out of them. Soon, Nina was a girl just a little more mature than Kim, with slightly larger breasts, and about an inch taller. The both of them marveled at their bodies.
"Wow, thanks Mom, we look amazing!" Kimberly gushed.
"Yes, but how will we explain this?"
Kim shrugged and took back the ring. "I wish I was nine." She immediately dwindled backwards through puberty, this time stopping as a child of nine. She looked very adorable.
"So the ring lets us become any age we like?" Nina was amazed.
"It seem so! I wish I was 2!" Kim dwindled down to a cherubic little girl and held her hands up. Nina laughed and picked her 'little sister' up, taking the ring from her.
"My this makes me remember when I used to feed you. I wish I could do that again." Suddenly her breast swelled, and filled with milk. "Oh!"
Kimmy laughed, looking at Nina hungrily. "Please?"
Nina sat down with Kim in her lap. "I wonder. I wish Kim was a baby again."
Kimmy's eyes got big as she shrank, returning to infancy. Then Nina put her to her breast, and Kimmy happily suckled from her. They had missed this, and it wasn't until she'd drunk her fill that Nina wished her back to being four years old. Then, she returned herself to that age. Experimentally, she made another wish. "I wish our clothes were repaired and we were wearing them again."
And like that, they were once again wearing their sundresses.
"I don't understand, the ring is magic?"
Nina nodded. "It can grant us any wish we want! What should we wish for?"