Anyway, I was wondering what my OC's mum would call him and his little brother as pet names. I'm thinking of doing a flashback when they're really, really small, say, three-and-four-ish

I can't speak for everyone but my mum came up with all sorts of names for us as we were growing up- usually related to our size. (If you look at the names Petunia calls Dudley you get the idea- although they're quite extreme.)

Some I've thought of off the top of my head are:-
'little chick' 'dumpling' 'sweetie' 'kitten' 'angel'

Or you could relate it to their names. I have a son called Charlie and have often called him 'Charlie Chuckles' - it's the alliteration that makes me think it's cute I guess.

Or maybe something they called themselves because they couldn't say their names properly when they were little. So Fred and George may have been Fwed and Gorg. Hermione coulkd have been 'My-knee'

Hope that helps.