Aside from the fact that flying is SO much quicker and easier, there's no way I'd eat $900 in nonrefundable plane tickets. For me, there would be no choice here at all. I'd fly. Sure you have to drive 3 hours round trip to the airport, but that's nothing compared to 20 hours round trip in a car!
It's perfectly natural to have panicky second thoughts after making a big decision. That doesn't mean you made the wrong decision. At this point, you have nonrefundable airfare for $900. Unless you're super rich with money to burn, that's not chump change. I can't believe you're even considering not flying at this point.
If the experience winds up being terrible, then you know for sure not to do it again. But you admit yourself you've never flown with the kids before. You don't know that it will be awful. You're just afraid it will be. It might be a million times better than driving. And even if the travel to and from Disney is horrid, it's not going to ruin your actual Disney vacation (unless you let it). It will just be something tedious you have to get through. Think of it as fodder for your blog.
But here's the thing: If you're going to fly, you have to do it with a positive attitude. Not with the attitude of "Oh no, this is going to be awful, and we're only doing it because otherwise we'd lose the money." If you go into it with a negative attitude, it will become a self-fulfilling prophecy. Kids pick up on our emotions, no matter how well we think we're hiding them. If you're nervous, scared, and unhappy about it, they'll feel the same way, too and it will manifest in their behavior (hyper or cranky meltdowns).
If you want this to go smoothly, you need to start brainwashing yourself right now to believe this is going to be FUN and AWESOME. Have you seen the movie the Secret? If not, go rent it right now and start learning how to visualize positive outcomes. And if your anxiety about flying is really FEAR, check this site out:
http://fearofflying.com/
It offers a free online video course to help you overcome your fear of flying.
I have flown over a dozen times since 9/11 without so much as turbulence. There are thousands more fatal car accidents every year than plane crashes. It's just that the occasional plane crash gets a lot more media attention. If that helps you put things in perspective at all.