http://www.jimmys-adventures.net/hiking-on-oahu-hawaii/

Should I visit Kauai or the Big Island when travelling with my husband and 1 year old son?
We’ll be visiting Hawaii over Christmas (Oahu) and will have time to spend 1-5 days on another island. Which island would be best travelling with a 1 year old? He doesn’t like to be in the car too long, but we’re happy to wear him hiking or whatever. Suggestions on activities to try with him there?
I should have said we’ve been to the Big Island before, but only for 2 days. We’ve never been to Kauai.
Without a doubt go to Kauai. You said that you don’t mind hiking, I don’t know about the Big Island but Kauai has some of the best view that you will ever see from your hiking trails. Also to drive from south shore to north shore only takes two hours (at good times). However I must say that 5 days will not be enough. I could go on a lot longer but if you get the ultimate guide to kauai (a small blue book sold almost everywhere on the island and on amazon.com) you will see just how much there is to do there. I hope this helps.
Hiking Diamond Head, Oahu, Hawaii