I don't mean to sound like a broken record, but... Whistler. Their kids program is second to none. We've had our children in every year since they were 18 months old (now 8 & 11). The instructors know how to have fun, the kids learn fast, and are smiling and don't want to leave at the end of the day. They give the parents a pager, so they can get in touch with you if the little ones need a hug from Mom & Dad. Blackcomb has "The Castle", a fun kids play area on the hill. Whistler Mtn's Emerald area has gentle runs groomed like butter, easy tree skiing for hide & seek, and a beginner terrain park (I love the Emerald area, but don't tell anyone). At the Olympic station they get snowmobile rides in the Whistler Kids program. For the babies and tots, there is a play room with sitters who are early childhood development students or graduates, and they get to have fun playing in the snow too.
Another nice thing about Whistler is that almost all hotels have their own pools, unlike some resorts that just have one community pool. I highly recommend Club Intrawest in the Upper Village. There are 2 seperate pool areas, one for kids and one for adults only. The kids one has a 20m water slide! There is also a video games room, and the games are free, as well as a theatre that has kids movies at 6:00. If they don't like the theatre, you can borrow kids videos and family board games for free.
It's almost a cliche praising Whistler, but the place really does have it all.