Peri's not a child, though. So it's not as sketchy. Granted, the age difference between the Doctor and any human companion is bound to be, er, quite a lot, but I find I'm much better able to explain that away as long as the apparent age difference (based on the actors) isn't terribly big, as long as the companion is old enough to count as an adult. (I don't ship Doctor/Companion a lot, but when I do, oh, I do.)

(I haven't really got a strong opinion on Peri, I haven't seen so much of her yet. Just wanted to point out my thoughts re: the sketchiness or not thereof of the shipping.)