Katy looks happy.

Katyusha needs a car seat! Actually, even in high school she might be just barely big enough to NOT need one!

... WTH does Shiho mean by calling Katyusha's mom "someone committing crime for fun"???

Best guess is that she treated it as a prank call in poor taste, not connecting her taking Katyusha home with what the mother was saying. As part of the joke, it's possible she didn't recognize Polyushka's voice, and so took it to be just some random prank caller.

I think Shiho knows who it is on the phone, but thinks that Katyusha's mother is prone to over-reacting to things. She clearly knows who Katyusha is and apparently knows how to get to her house. (It seems like it is a very short drive if they can only get a few sentences across before Shiho arrives at the house.) My guess is that the "committing crime for fun" thing is related to how it looks to some people like Katyusha's mother is stalking Katyusha, as in the last frame of post #4361594.