The driving all night idea is great if you are up for it....I love driving at night and when my girls were little they would sleep right through most of the drive. My DH HATES to drive at night...so we could never go all night....sigh, but if you all are up for it try it for the first trip and see how it goes....if it is awful then you can have the driving trip behind you and fly the second trip....besides, less chance of bad weather for the first trip anyways.
I highly recommend either McDonalds with playplaces or picnics at roadside parks/rest areas instead of sit down meals while travelling....giving them a chance to run and squeal and wiggle sure seemed to help a LOT. We usually avoided McD's like the plague, but those play places were very welcome on long road trips!! Roadside parks were great too....picnics with food from home that they liked and they could run while they ate!!
We always got a lot of mileage out of new coloring books and crayons or markers....a small dry erase board with bright markers was also great for keeping them entertained (actually anything that was new always seemed to hold their interest longer on the road....I always made a run to the Dollar store and got things I thought they would like and kept them tucked away until trip day and then got them out 1 thing at a time as needed.
We also got a lot of mileage out of childrens books on cassette tapes (I am dating myself....I guess they are all on CD's now....those were especially great if their was an adult in the back seat with them to hold the book and turn the pages....but even if the book wasn't even open they liked to listen as we drove.
We dreaded the first long road trip with them (it was 17 hours from where we lived then), they had just turned 2 and 4...(and the 2 year old was not usually a good traveller) but DH and I still remember how amazed we were that they did so well.
I think by thinking it all through and planning well the trip will be easier than you think. (and you are already doing that).
Have a GREAT time.