The Blacks, being such caricature of victorian gothic tropes and lifestyle, could have dressed their infants to the victorian fashion (all in dresses until they're about 4). Did any of the marauders ever come across a picture of baby Sirius in extravagant frilly baby dress?
LOL rip......... realistically I think it's unlikely the marauders would ever see such a treasure because 1. i don't think the blacks would have a wealth of cute family photos and baby pics. they'd have posed for a super serious sombre family portrait* once and I think they'd actually dress their kids very soberly in robes for the purposes of a portrait.
*quick visual representation of what i mean. and by quick i mean i actually spent 2 hours on it because it was meant to be a sketch and i got too locked in
I do think wizarding kids would wear dress-like things, though at the end of the day robes are also very like dresses. so the child version is also like a dress. i have a theory that the 'smock' thing that sev was wearing when he met lily was actually eileen's childhood clothes
and 2. even if there was photographic evidence the marauders would never have visited grimmauld place to see it. but if they had they would undoubtedly given him a lot of grief.
