We've driven 2x to HHI with our kids 3, 2 & 5 months (21 hours). It was long, but the kids did suprisingly well.
That said, I'd fly. We didn't mind the drive to HHI because we were going to relax & do nothing while there. I can't imagine the long drive and then having to "do" wdw. My theory is that it could be a flight from hell, and a lot of stress...but it would be (in my case) 4 hours of chaos...but no matter how hard the prep/flight/transfers are...it's over with fairly quickly (5 hrs tops) for us, and then we can begin our vacation. If you drive, you could possibly drag that chaos out for a few days before you really get to start vacation.
Karen