Been to both. Didn't take kids either time. Well once I was a kid (16) and the other time I went with my hubby at age 22.
Oahu seemed to have LOTS to do for kids though I was miserable because I was with my rotten little brother (9 at the time... which is not fun for a 16 year old).
But I remember lots of tours and visits to museums, Hanama Bay (sp?) snorkeling -- now that was fun!, and a trip to the zoo.. also fun.
Maui was fun but less touristy. If you stay at a hotel on the beach with plenty of services, your kids would probably be fine. But it was a long drive to Hana to see the 7 waterfalls (and splash in the them), and the long early morning drive to the top of Haleakala to see the sunrise... but they were both spectacular.
I saw the best whale watching I'd ever seen on Maui and I loved the Iao Valley of And kids would be bored, I think. So it depends on your kids and where you're staying. If they need constant activity and like glitz, I'd go for the more touristy and go to Oahu.
http://www.visitmaui.com/
http://www.visit-oahu.com/
PS if you have asthma, watch out for the burning of the sugar cane fields on Maui. I think we were there in the Spring, years ago, and they were burning the fields, which made me ill and we had to pull over and I got out of the car just in time to throw up at the feet of some old nuns that were walking beside the road.