The kids have to be written well. And, of course, the adults, but I hate with a fucking PASSION smarmy, precocious, perfect wee little adults in kid's bodies. Of course I also hate the OMG REBELLING I HATE YOU PARENTS!!!! kids too. There's a medium, and it has to be reached, and there's a million ways to do it, but everyone seems to gravitate to extremes.

Also it has to be believable. With Ray/Fraser, yes, it's just about inevitable that they would have kids, one way or the other. But with other pairings, it's just... no. And the REACTION to the kids has to fit. I read the most wonderful X-Men kidfic once, with Beast having a little girl, and it was charming and good and nice and yay, but the way he intellectualized everything, and paniced in his calm sort of way, that's what made it.

So, if it's good, then it's good. If it's not, then it's fucking awful. And I don't even like kids.