You plan sounds very good to me. You're getting a 'taste' of the parks and I'll bet you'll want to return for more!
I would eliminate trying for a boat ride on Lake Powell/Rainbow Bridge. You don't have enough time.
Ruby's is a good place to stay for Bryce. The free shuttle stops there and it has a good laundry and restaurant. Definitely try to walk down a trail in Bryce if you're all able. Wall Street is preferred but a tough one up! It's like a fairy wonderland down there. :)
Definitely move to Zion rather than staying at Ruby's to tour Zion. Either stay in the park at Watchman campground or just outside in Springdale, UT. The free shuttle makes stops in Springdale. You'll need to use the shuttle anyway to tour Zion - no vehicles allowed. Get off the shuttle and let the kids splash in the river.
I think you'll all have fun on this trip. It definitely takes a lot of planning, doesn't it? You just do what you can in the limited amount of days you have. It's better than not seeing these parks at all.