Plan for the worst and hope for the best. That way, worst case scenario you get an extra day or so at your destination and don't feel stressed along the way.
50mph is a good planning average to use assuming you don't futz around at the gas station/pee break stops too much. With kids, it'll honestly be slower. We lose 30 minutes minimum at any stop if we (2 adults) go to the bathroom, let the dogs out and fill up gas.
My guess is that it'd easily take you 5 days to get there with 8 hours of driving each day and no sight seeing. I wouldn't take that trip unless I Was planning to be on the road for a month or more. Figure a week of travel on either end and then 2 weeks of vacation in the OBX. Any less and it wouldn't be worth it to me unless we were stopping and enjoying spots along the way as part of the vacation and not using the 5 days of travel time as work to get to the OBX. With kids, you're probably looking at 7 days of travel each way, so you can make time for something else besides sitting in the car.