For us, 3200 miles round trip is too much for us to do in 9 days. We would use at least 7 of those days driving.
Since you have the possibility of flying the family out, that would change the equation, especially if they flew both directions.
It's a two-day drive from the Grand Canyon to Big Bend National Park, so we wouldn't do both on the same 9-day trip. - Distances are more than they seem in the west. I remember the first time we were told that the grocery store is just a short distance up the road. It was a two-hour drive. (This was in Montana.)
How old are the kids?
You could elect to do both the South and North Rims of the Grand Canyon - 200 mile drive. Petrified Forest is 200 miles away. Hoover Dam is 250 miles away.
As far as off-road, I'll defer to others.
Tom
