I disagree for not enough things for kids to do.
See my travel log on Kauai at www.tabhauser.com and click Kauai.
(this is my personal trip log with no advertising or signing in)
The kids will plenty if the parents show them what is around.

The Big Island. What kid can resist a volcano? I would not rule out Kauai which has the only navigable river among the islands and a very coolFor 2 islands I would suggest at least a 10-day trip.

We missed out on this tubing tour on Kauai last year but is sure looked fun: http://www.tombarefoot.com/kauai/specialty_kauai.html