Then, there's the issue of sleep. How well do the kids sleep on a long car ride? Think you and DH are going to be able to sleep all that well when not the one driving? Everyone is different, so you have to evaluate your own family... but I would wager everyone arrives in Disney feeling quite tired, and the better part of the first day might be a bit of a wash... especially for you and DH after driving. Also consider these effects on the return trip. I know in my own experience, after that long a drive, there's no way I'd want to have to be back at work the day after that long a drive.
Not sure what the OP's schedule is like, but we take being tired from driving into account and that's why we do longer trips (usually 12 nights at Disney, and sometimes a few extra days at the beginning of the trip visiting family in FL).
We drive straight through (about 23 hours). DH usually does almost all of the driving. We usually leave in the afternoon/early evening so we arrive sometime late morning the following day. We plan that as a day to unpack, get some groceries, check out the resort, and rest. DH will take a nap while I walk around and/or swim with the kids, and we all go to bed early so we are ready to hit the parks for rope drop the next morning.
Every trip we have planned to leave for home on either a Thursday or Friday morning (sleep in a bit, but get on the road by 11) so we arrive home Friday or Saturday morning. This gives us about two days and at least two nights' sleep in our own beds before we have to return to work/school.
Honestly, over the years we would not have been able to afford all our vacations if we didn't drive (family of 5). Since we don't mind the drive and we have the ability to go for longer periods of time, now even though we could "afford" the plane tickets we would rather just save that money toward another vacation.
However, if we only had the ability to go for a week or less it would not be worth all that driving, being tired, and waste of those precious vacation days so we would likely justify the cost of flying.