Mark grabbed the coin and said "I wish I was two years old and Rebecca's little brother, still keeping my adult mind."
He felt himself shrinking and his legs, arms and belly became a lot pudgier with baby fat. Looking down on himself, he noticed his shirt and diaper shrank to still fit him, but otherwise looked the same as before. When he looked back up, he saw Jesse put Rebecca back on the ground from the bench, now wearing a fresh diaper under her dress and looking a lot taller than Mark. When she saw him, she ran towards him yelling "I love you Marky!" She hugged him tight, his eyes now on the level of her chest.
"You big Becky!" Mark said hugging her back.
"Yeah, I'm your big sister. You know, I'm really strong. I can lift you up just like Mommy does!" Rebecca said.
Mark didn't quite believe that, but she actually started to pull him up in the hug. His shirt was pulled up, exposing his diaper even more, and he laughed happily. To his great surprise, he was actually lifted a tiny bit off the ground.
"Look Mommy, I can carry Marky!" Rebecca called out to Jesse.
"Whoa, you're really strong! But be careful!" Jesse called back in surprise.
Rebecca dropped Mark back to the ground as she could only support his weight for a short time.
"Why did you make yourself younger?" she asked him
"Well, it's true I was a bit old for diapers and of course people would wonder why Mommy's not potty training me. Now wearing diapers at all times is perfectly normal. Also I want to know what having a big sister is like."
"Oh, ok. Maybe I should become younger too. But on the other hand, I like being a big sister. I feel strong and proud and I can help and protect you."
"I can make you a bit younger if you want, but I still want to be younger than you. So you could be two and I'd be one year old. Do you want that?"
Rebecca considered Mark's offer. She decided to...