Haven't done one -- but looked into it once. For the specific sailing locations I looked at: Cabins are limited to 2 occupants -- which might require a second cabin for your family. The RCI information lists an additional fee of $895 per person not including tips. Suggested tip ran ~$100 per person.
After responding I checked the reviews over on The Timeshare User's Group (tug2.com). They are many reviews, very detailed trip reports, lots of great info. One mentioned that the fee was worth every penny!
(Consider: There are several different Tradewinds Cruise Club sailing locations. The ones I looked at showed $895pp ... perhaps this varies by location??)