Jim,
You are setting yourself up for what we have taken to calling a Bonzai Run...
We have done 1100 mile days in our youth (lucky to survive) and 600 mile days are not that hard on the now two of us even now. But it is not a suggested activity.
If you do not have two drivers, it will be very hard. If you do not have a navigator on duty all the time it will be hard. Teach both kids to be navigators. The navigator does more than watch the GPS. If you travel with a laptop (suggested), and wire (suggested) the navigator can plan fuel and other stops long in advance and save a lot of wasted running time and undo a lot of what is otherwise burden on the driver. The off-watch can relax in the back.
A lot will depend on how comfortable everybody is. Do not underestimate that requirement.
Do Not drive tired. That is a real easy way to end up dead.
Matt