Well, I have no idea if there's a fandom-wide theory on it, but for me, personally, kid-fic has always been any fanfic which has kids in it. Which would cover all of the above.
LabRat
Athos: If you'd told us what you were doing, we might have been able to plan this properly. Aramis: Yes, sorry. Athos: No, no, by all means, let's keep things suicidal.
Which would cover all of the above. 
