We have several family groups, and groups consisting of 2 or 3 families, who return year after year.  If you rent a whole cat, you will pay the lower of the full-cat rate, or the individual rates (with kids up to 19 only paying $199, it can be less than the full-cat rate).  If you rent a full cat, you can bring children of any age, as long as they are capable of skiing deep snow in the backcountry, and you ski with them at all times.  Children under 12 are discouraged unless you have your own private cat, for social reasons.