Smooth Operator/Powell Home/Milly/Straight to sex
So, Milly is literally here to have sex with you, huh? It makes sense. She may only be 7 years old, but she's a mouse. It's actually normal for mice to be fertile at her age, and from what you've learned from Matt, any girl is perfectly fine to have sex with so long as she's old enough to have babies.
Her and Mai have been hanging around you for a couple years now. They'd made the excuse they were there to play with Tara at first, but it's pretty obvious Mai was mostly just interested in having babies with you. She's even talked about wanting to have your babies on a few occasions, but was dissuaded when she was told you're not old enough to make babies yet. As for Milly, she was just always following the older girl in everything she did since she was a couple years older. It was more like Milly was Mai's easy in with Tarra in order to get closer to you.
The only mystery is why Mai is hiding in the closet still. You catch that the door is cracked slightly open when you glance in that direction only for it to be quickly pulled back closed, showing she was trying to avoid being noticed but was definitely curious to see what you were doing with Milly.
Oh well, you'll just let her watch while you have fun with her little 7 year old follower here.