We just stayed in Yellowstone for 5 (had 7 reserved but left early) and 5 in Tetons.
Good advice above, but you'll feel slightly different based on your own taste. I think 2.5 days at Tetons is fine if you are a "see it and run" family. If you are okay spending your time slowly and soaking things in, then Tetons could occupy much more time. We hit it perfect for us at 5 nights, but there are many things we didn't do. We could go back (and will) and do a different list of activities.
We stayed 2 nights at Gros Ventre and 3 at Colter Bay. Staying in both works out, as you can eliminate some driving. We hiked to Inspiration Point, that was our only hike. I don't hear many discuss it, but we loved spending a day driving "River Road". It is a gravel road that follows the Snake River for about the length of the park. We saw two other cars the entire day. We brought a picnic and spent time just sitting out in the sage brush watching wildlife and the rocks...the rocks didn't do much, but they sure are purdy. I agree with those that can sit and look for hours. They mezmorize you.
You didn't ask, but