If they need to fit a family of 8 in weekend stays, planning far in advance is key. Since their kids are in many activities, chances are, they will need to cancel/reschedule many of their stays. My daughter was busy every weekend all 4 years of high school - they'd even throw in "mandatory" practices during all holiday breaks. At that time, we couldn't do any weekends away. We could do one planned vacation, which was planned well in advance and she was "excused" because it was planned months in advance and was such a rare thing. Not sure if that family will have as rigorous of a schedule as my kids did in high school, but at least my second one had summers and true holidays off.
With WM, when you book 13 months in advance, you have to book a minimum of 7 nights. You can book shorter stays 10 months out, but any popular places will be booked and they will need to use the wait list. There is some work involved in getting the popular reservations. They may be better off renting from owners for a few times to see how it works out. There are plenty of owners who have too many points and they will gladly put in a wait list for someone else and/or log into the site many times per day to try to grab something.
EDIT TO ADD: They might also want to check out bypassing timeshares altogether and stay at Embassy Suites or Homewood Suites. They are very similar to timeshares, but you get a hot breakfast every day and daily housekeeping. Embassy has free cocktails in the evening. Homewood has full kitchens.
