Having done every island in Hawaii more than a few times, if this group has an ultimate destination of Princeville for their entire vacation, I'd say just get there in one day. Kauaii is a very easy place to drive, there's only one road around the island. The group is not elderly and can certainly split a 45 minute drive straight to princeville. It's not like a long flight saps your strength as if you hadn't had even a sip of water or morsel of bread the entire trip.
There is nothing in HNL that warrants you going out of your way (IMHO), if you are going on a single island trip. The hassle of the 25 minute trip in two directions to the Moana is equal or worse than doing it all in one day. The Moana's pool area is so small and the beach area so narrow that it's basically a waste for one night just to break up your travel.