I've not sailed on this ship, but in an effort to answer your question I just read a review of the Royal Clipper from the October 2005 edition of Cruise Travel magazine.
This is a real sailing ship. There are no youth activities normally scheduled, and there are no youth facilities. The reviewer also noted that the ship has a much more pronounced motion than a normal cruise ship. In other words, the reviewer got seasick. There are no elevators. Getting around the ship requires gang planks, ladderways, and steps.
So - to me it would not appear to be a good idea to have the kids go. Actually it may not be a good idea for the parents either, given that you said they are not very active and in their 80's.