sure it's been pondered over millions of times already, but I'm doing it once more!
Drawing some recent canon art has given me a lot of thoughts and re-thoughts on clothes, fashion and style in the HP Wizarding World.
I consulted the HP Lexicon for reference/research.
So it's established that wizards, for the most part, tend to stick the the standard outfit: robes, boots/shoes and cloaks. Accessories like hats, gloves, scarves, earmuffs and handbags are normal in both the wizard and the Muggle worlds. I don't quite recall, but I think in GoF the Beauxbaton girls were also sporting armmuffs. Or maybe that was just my imagination.
There are exceptions to this standard outfit. Fudge for example, wears trousers. Whether or not this is just a Minister of Magic accoutrement as a liason between Muggle and Wizard world, I prefer to assume that other wizards wear trousers as well. They are likely not Muggle-style jeans or cargos, as it seems wizards tend to wear much brighter and extravagantly designed clothes in general.
And by this reasoning, I'd also like to think that witches do wear skirts, or at least perhaps shorter, thinner material robes on hotter days.
Finally, I'm assuming that due to weather changes, robes can also be layered. On a cool autumn day, Snape for example might layer his robes for school. He'd start off with underwear first, and then maybe a thin shorter robe (like a nightie). Then his normal long robes, which on warmer days he could wear by itself. Then an overrobe, possibly with clasps (the equivalant being Muggle blazers or cardigans). And finally, for traversing outside, a cloak. Too bad he forgot his boots, oh noes frostbite!!!
So what about the kids?
It says in the Lexicon that the kids and younger adults will for the most part wear Muggle-ish clothing on casual days. But I don't think this is entirely the case. For people like Dean, Tonks and the Creeveys all having Muggle parents, sure.
But how would pureblood children wear Muggle clothes, if their parents don't even know that nighties and galoshes are not everyday Muggle wear? Where would Gran Longbottom even start trying to shop for Muggle clothes? And can you imagine Draco lounging around in a vintage The Young Ones t-shirt and factory-distressed black cordouroys? (well, of course we can because it's so emo, but that's not the point, heeheh)
So like, say the Trio have a weekend to Hogsmeade. Hermione could possibly be in a nice cotton blouse and cargo skirt that she bought at, say, H&M. And Harry would be in some Duddie-jeans and a Duddie-tshirt (the boy doesn't have much of an imagination in the fashion department; why doesn't the poor socially-inept child go an a bloody shopping spree in canon? Why didn't Parvati and Lavender have a Fashion Intervention? Oi.) And Ron? As a fullblood wizard, Ron'd be wearing something completely wizardly.
Ron would wear casual robes, then? Perhaps, but if jumpers and trousers are the norm in the wizarding world, that style of clothes - except completely un-Mugglelike - would be acceptable. So. What would he wear, that's distinctly wizardly and completely un-Muggle? Children's casual wizardly clothes is something left up to the fanartist's imagination.
I can think of a couple exceptions to this pureblood dress:
1) Bill Weasley, who dresses like he was going to, as Harry put it, a rock concert.
2) young!Sirius Black. While not established in canon, I can see teenaged Sirius gleefully trying out Beatle boots, bellbottoms and colourful striped polyester shirts with wide lapels.
Why these two? Because they are both purebloods and they are both, to different extents, rebellious. Bill is a rebel in that "Dude, I'm so cool. But I still love my folks!" and; and Sirius, rebellious in possibly every way.
However, most fullblood wizards tend to stick to their own and consider Muggle anything to be odd and strange, including their clothes.
Hmm. Anyway, I'm purposely not mentioning the movies because they are not my canon. With the exception of the Quidditch uniforms, because I LOVED those designs, ahah!
Okay, and I did rely on the movies' British school uniform designs in my fanart as well, but I'm trying to break out of that lazy habit and utilize my own imagination. There's nothing wrong with the movie designs, but it's just not my thing anymore; it's a personal choice.
Despite all these various difference, at least one article is clothing is firmly established as an unchanging crossover between Muggle and wizard clothing. I'm referring of course, to socks. Pretty eiw, but there it is!