Leave at nap-time, or if you don't mind driving late, leave in the evening. My parents are 6 hours from me, and I would leave about 4pm, stop between 6-7 for supper, and be there between 10-11pm. I had 4 at the time, and the youngest 3 were each 2 years apart (so at one time - 2,4,6,& 10).
I found that the younger ones would take a nap, and although they were refreshed when we got there and I was tired, Grandma and Grandpa were more than ready to have some play time with the grandkids (esp since they didn't see them very often with the distance).
Take the pump with you. Keep him on as much of a regular routine as possible. Also pack any special blankets, teddies, etc that will make the kids feel more "at home".