Quebec City is an interesting place from my perspective but I don't know that it's a slam dunk for 12 year olds, especially if they don't have a good working knowledge of French. Not to say that it's a bad choice as I'm sure some 12 year olds would love Quebec City, but I think Montreal might be a better option being a bigger city, more bilingual (and therefore accessible to English speakers) and more options all around.

And I love Chicago, personally with my kids (8 and 10 but experienced big city travellers, our last 3 trips were to London, NYC and LA), I'd probably rank the destinations I've mentioned as 1. Chicago, 2. Montreal, 3. Quebec. My 10 year old is a junior hockey freak and Quebec is a great junior hockey town, so even though it's third ranked I know at least one of my kids would be going nuts over a trip there, haha.