Ohhhh, that makes a lot of sense. If there's just too much cutesiness, I can't take it. I think that dS is the only fandom I will ever like kidfic in, mostly because their circumstances - staying together - tend to allow for that. And it's nice to think of them not being totally alone and isolated up north somewhere, if that makes sense. You said it:

I can imagine them both wanting a long-term, secure, domestic, spouse-and-family situation, that creates something for the future, and raising kids sort of pings at that.

Precisely. (And ditto also to the emotional payoffs for both Ray and Fraser. Oh, it just gets me where it hurts.)

It's also a realistic setting for both their teamwork and their friction to play out, with lower stakes than crime-fighting - of course mom and dad argue but they still love each other, right? Or something.

Yep, makes sense to me. They have such a wonderful, interesting dynamic - they're not perfectly amiable, they bicker, but they clearly love each other, and are willing to work for it - that them having a kid/kids could work out, in the fantasy of them as real people, so to speak, and in fanfic.

So, yeah. F/K kidfic is the only kind that's worked for me so far. Then again, I've only been in two fandoms and have only had two OTPs. *g*