I think with kids that age 600 miles a day is very doable. Even with stops for fuel, potty (obviously my kids are still young...), food, etc. you should be able to knock out 600 miles in maybe 11 hours. 12 tops.
Back before we had kids we used to crack off 750 miles to visit my in-laws in 12 hours flat (granted that in a car). A couple of months ago we drove from LA to Boise with our kids (6, 3, 1) and our fifth wheel in one 15 hour stretch, and that was 900 miles. I got everybody up at 2:00 am and loaded them in the truck so they slept the first 4 or so hours before we stopped for breakfast.
Bring iPads, DVD players (stop at Redboxes along the way), books, etc.
