As a family of 4 with 9 year old children, we did the following:
Zion: 5 days and really, really, did NOT want to leave. We were hiking from sun up to sun down all days and didn't see all we wanted.
Capitol Reef: 2 nights and would have liked 3. But I didn't see need for any more than that.
Bryce: 2 nights. Why oh why did we give this one only two nights??? Between hiking and star gazing, we were on the run from sunrise to midnight or later. There were still several things we wanted to do.
This being said, we have friends that say they went to Bryce. They spent 2 hours there. Seeing the main overlooks or doing the drive through is kind of like looking at a snapshot. You can't "experience" the park that quickly.