[Kipper is King]

Go hiking around Sedona. The rock formations around there make it feel like you're on another planet! (Check out Bell Rock and Cathedral Rock) Also, it's a shorter drive from Phoenix than the Grand Canyon. If you do go into Sedona itself, be aware there is only one official tourist information office. The rest reeeally want you to buy real estate! :-)

I should add that much of the "hiking" around there is actually just short walks. It shouldn't be too intensive for younger kids.