We usually stay for 7 nights as that is the standard stay length for a timeshare owner, and it's not too long. It generally feels too short, so we are considering doing two weeks next time.

Going out and coming back will take a while, figure on an hour round trip at least. Going to Honolulu or Waikiki and back will probably take two hours round trip.

There is a lot to do at the resort as it is not just KOBC, there is Disney Aulani and a Four Seasons both within walking distance - I assume the little ones will be in a stroller. It's about a 10-15 minute walk to the Four Seasons for grown-ups.

I don't usually do airbnb, so don't know anything about it, but I would think you should be able to rent nights on eBay, and it will likely be less expensive than through Marriott. WIth the Vacation Club points-based system, there are owners who will book rooms and rent them out via eBay.