In November, on our guided hike in West Maui Mountains (via Attitudes 'n Latitudes, which I highly recommend), we met a couple with young kids (ages 5 and 7?). They had just spent a week on Big Island. I asked them how they liked it and they said that they didn't care for it. They really enjoyed the activities that they did, but the father said getting there each day was exhausting. His comment was, "The Big Island is...too big."

They all agreed that they wished they had spent their entire time in Maui. It's large enough to do a lot of different things, but compact enough that you're not spending your entire trip in a car. (Keep in mind that these parents were pretty adventurous. The hike we were on traversed over a very steep gorge and involved using tree limbs to use as hand and footholds.)

Personally I think Big Island sounds great for adults and for older kids who can amuse themselves for a few hours (e.g. give my 15/13 year old kids an iPod and they're good for several hours). But to take young kids who've just spent an eternity on a plane and ask them to sit quiety in a car for several hours a day can be very tedious.

When I read that they loved Oahu, my first instinct was to say go with Maui. IMO, Maui is the middle ground between the urban pace of Oahu and the country pace of Big Island. It's the suburbs of Hawaii.