Slow and Steady Wins the Race
Welcome aboard the RoadTrip America forum.
From Philadelphia to Las Vegas (a good proxy for your two destinations and becoming more family oriented all the time) is just under 2500 miles. If you were to take 6 days to make the drive each way, you'd only have to average about 425 miles a day. At that pace you'd have time to get out of the car a couple of time a day and explore some of the sights along the way. That would go a long way to keeping everybody interested in the trip, especially if during the planning they each got to pick a couple of stops that particularly appeal to them. You can even take different routes out and back to maximize the new things you'll be seeing without paying too much of a penalty in time and distance. Such a schedule would then leave you 2 days, one to have a good long look at the Grand Canyon, and one for Zion. I think you could also squeeze a few extra days by using the 'extra' weekend at either end of your two weeks to give you 16 days instead of 14, or you could, by pushing a little harder and averaging 500 miles a day, make a one-way crossing in 5 days.
AZBuck