I took my family to the Grand Tetons, Yellowstone, and Glacier National Park two summers ago, and it was easily the best trip we've ever taken. My kids absolutely loved it, as did my not-so-outdoorsy wife. We spent a couple days in Yellowstone, and the majority of our time in GNP, camping.
My favorite sites were:
1. Grand Prismatic Spring - if you have time, take the walk around it as well as the walkway that goes up the hill above it...you'll have to park 2 different times, which is a pain, but well worth it.
2. Grand Canyon of the Yellowstone
3. Driving the loop to the NE side of the park