The thing is, Rosalyn is really a rip off of Mamimi from FLCL (and I also steal a lot of Naota's traits for Calvin in Season 1), you see her have this hugely inappropriate relationship with Naota despite their age difference, and it's presented as either humorous or endearing. You see this a lot in anime, a young male having a pseudo-romantic relationship with an older female and it's nobody seems to realize how creepy it is.
So I figure, why not actually play it as creepy, showing that Rosalyn really is taking advantage of Calvin and he's negatively affected by it, but since he's so isolated and socially anxious he doesn't really have that many other people to hang out with, so he keeps going back her even if she frequently makes him uncomfortable.
We're also ripping parts of that girl from Eternal Sunshine of the Spotless Mind, especially when Rosalyn initially starts babysitting Calvin and comments on how much she loves the decor of the house. When Hobbes starts complaining about Rosalyn Calvin reminds Hobbes "It was you who told me to make friends."