We were in Whistler last Xmas and it was amazing. Busy busy on the main holiday, but really great. .. we're going back at Easter!
I did a lot of research about possible weather, snow levels and facilities and decided it was the one. Banff possibly too cold in a cold snap (it was -40 last Xmas, whistler was also unusually cold too, but not so bad). I've been to Breckinridge and it didn't offer enough difference to go back again. Vail didn't seem quite as snow sure either.
My DD5 needed a day off skiing and we used Whistler Nannies. They were great, flexible and great service. I would contact them. I'd hurry up though it's peak time and you'll want a block booking.
The kids ski lessons are brilliant too. All day, very technical and on a GPS monitor so you can look at their stats. They take skiing very seriously and it's max 4 in a group. They pay real attention to skills and give feedback on all aspects. Much much better than 10 trailing behind an instructor, but expensive because of that.
Canada doesnt groom the slopes in quite the same as Europe so you might find some of the skiing quite challenging. Greens and Blues can be Green/blue/red equivalents in Europe. Black diamonds are usually ungroomed so moguled to fuck. There is usually one or two groomed though. Double blacks are cliffs - I kid you not! The kids lessons will take them down diamonds and it will really push your 11year old on and give them a moguls master class.
Have a look at Creekside rather than Whistler Village. It's cheaper, quieter and the same hill. The Peak to Peak gondola means you can transfer easily between both mountains (although the queue can be big)
I've been in Les Deux Alps at -25 at Xmas, I've been to Whistler and it was the same, possibly lower. The range is usually no lower than -15, so I'd say yes, it could be colder than Europe, but that depends on which resort you compare it to. Banff and Lake Louise are noticeably colder as further in land. The ocean keeps Whistler warmer.
The real estate is bigger in North America so rooms are bigger, standards are higher (US clients demand it), service is good, queue are unfailing polite and people chat to you on the lifts and share their local knowledge. I think it's worth the flight... whether I'd do it with a one year old, I'm not sure as an 8 hour time shift takes a bit of getting over. We went for 13 nights and did other activities too so no one got fed up with skiing - Tree trekking, bobsledding at the Olympic track (amazing fun), ice skating, Coca Cola tube park and the kids coped well.