When are you traveling to Maui? If it's low season (e.g. the fall), you have a much better chance of being able to rent at WKORV or another condo for 2 days. If you're traveling in high season, you're going to face more limitations because many condo owners impose minimum number of nights, so that they're not left with lots of hard-to-rent partial weeks during their bread-and-butter season.
As far as renting at WKORV, if you rent the north side, there's very little room to squeeze 3 teens into a 1 bdrm unit. Our teens used the fold-out couch and once you dismantle the bed, there's no room left to put a cot or air mattress. You might be able to put a twin air mattress in the hallway in front of the washer/dryer, but that's really packing them in.
There should sufficient square footage in the south side to put a cot, in addition to the fold-out couch. Whether the hotel would provide a cot is another question because 5 may exceed their maximum occupancy.
Sometimes you can find better good deals for WKORV at Wotif.com. I just checked rates for January/February, and they didn't beat Starwood's rate, but once again, if it's low season, you might be able to score a better deal.
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Personally, I'd consider heading to a different part of the island, such as Kihei or Hana, for those last 2 days. There are many affordable condos/hotels in Kihei, just north of Wailea. Or, better yet, the morning that you check out of WKORV, plan to do the Road to Hana, and take the entire day to enjoy the sights (pack a lunch). Hana is an entirely different culture than the tourist meccas of Ka'anapali and Wailea. And the kids will have a blast on Hamoa Beach, which has awesome body surfing.
Looks like you can rent a 2 bdrm at Hana Maui Kai for ~$450/night.
http://www.hanakaimaui.com/resv1.php
