I can tell you to AVOID Snowbird (Utah) and Winter Park (CO). The former has limited greens which would be blues at other resorts. There is also a lot of traversing to get to the lifts, and EVERYONE uses the bunny slope to get from the hotel to the lift/tram. Alta, which is just down the road, has much better green slopes. (I speak from the experience of someone who went to Snowbird her second time skiing, and spent an evening crying after a terrible, frustrating day.)
Winter Park does have nice green slopes, but again, EVERYONE uses the same slope at the end of the day, and people are careless. There are lots of day trippers from Denver, so it gets quite crowded near the train departure time on the weekend.
I think Steamboat Springs had the best variety of terrain, and because of the tram and many advanced side runs, you weren't dealing with as many experts among the beginners. The ski school was excellent, and has wonderful child care.