In The Goblet of Fire, Uncle Vernon asks Harry what the Weasleys will be wearing when they pick him up, and that he's seen what wizards wear.
'I've seen the sort of stuff your lot wear. They'd better have the decency to put on normal clothes, that's all.'
Harry felt a slight sense of foreboding. He had rarely seen Mr. or Mrs. Weasley wearing anything that the Dursleys would call "normal". Their children might don Muggle clothing during the holidays, but Mr. and Mrs. Weasley usually wore long robes in varying states of shabbiness.
So where would the Weasley children have learned about Muggle attire?