Something else to ponder: Many DVC members rent their points when they know they'll be unable to use them. Your parents could concevably buy a certain amount of points to cover the use of a smaller room when the kids are all grown up, and rent other people's points if/when they decide to book a grand villa. I only bring it up because as everyone gets older, you all might not be able to go on vacation at the same time. So unless they're willing to dole the points out to the various kids when their needs arise, there's always either going to be times when points need to get banked or unused (if they don't rent them themselves) OR they'll need more points. They should think about long-term needs first, will they keep going as the family gets older and moves out, and will this family of 10 always go to WDW together?
My advice would be, if they can afford it, to start with 200 points. They could always bank the points one year, and then borrow from another year, giving them 600 points every three years. If they wind up needing more, they can buy points or rent them on an as-needed basis.
