Ouch. Their reaction must have hurt, and like many posters I'm not sure I would say you did anything
wrong even though I understand why it might feel like it. And do keep in mind that their behavior seems to be a bit out-of-character for them, at least according to your later post.
First, silly reactions!
I have to admit that I had this knee-jerk reaction:
I'm sure many of their friends would be more than happy to go to WDW in their place. Maybe you could child-swap before the trip occurs?
Or I volunteer to come along and be a DD for a week (I'm a bit older than your children, but I act young for my age); I'll even pay my own airfare and bring my own spending money!
Or perhaps you could put off your plans for another year, which would permit them the opportunity to earn their spending money? And then they'd have a whole year to anticipate the vacation!
More seriously...
I do think that all your children are of an age where they could benefit from learning about how your household finances works, about the value of money, and about how they can be expected to contribute to the operation of the home and family. By this, I mean to say that I believe children need to learn how they should contribute to the smooth operation of the household, not only financially or in terms of physical labor (though I say "yea" to both) but in other ways.
In terms of your upcoming trip, your children can contribute their allowance/gift/discretionary money. And they should be involved in the labor involved, from laundry & packing to keeping the hotel room clean. But there are other things:
1) They can help plan the trip, within the constraints of the family budget and with the parents retaining both the list of original option and veto power. I don't think it's too early to talk about $ decisions with your children, about how the choice to buy one thing, like a vacation (big) or plush animal (small) means other things are sacrificed. Also, I think children can realize that these decisions affect all members of the family, not just the individual child.
2) They should adopt an attitude that makes the trip something to look foward to (they are responsible for the Disney pixie dust as much as you). When they do, it demonstrates their respect for you. They could help with countdown calendars, planning travel activities, volunteer for all the little extra chores that go with vacation planning, and I would especially suggest they make you thank-you cards.
I also agree with several people's comments, excerpted here:
bunnysmum said:
I like the idea of having the kids take more ownership of paying for the trip (on a small, kid-sized scale..meaning they contribute to their own spending money). [...] Maybe you could have a family meeting and discuss the fact that they probably don't realize how you (parents) have scrimped and saved to make this trip possible, and that there simply isn't spending money available because the car had to be fixed. You could tell them that the family has a choice...either go to Disney with less spending money, or cancel the trip.
Friendly Frog said:
It is never too early to teach children about money. Instead of a free allowance, allowance can be their pay for the chores they do. You can assign chores for ANY age child. [...] As your kids get older, they can do more chores for more pay. This teaches them lessons in life that can not be taught just by telling them. (You will find that those " I need that" items decrease when they have to pay for part or all of the non necessities. )
graygables said:
I would suggest you be honest with your kids. Tell them you made a mistake handing them a hundred bucks last year and you won't make that mistake again. Explain (again) the financial situation and the concept of family (making sacrifices for the happiness of others) and gratitude (a thankful heart is a happy heart). If they are old enough, I'd suggest having them start a gratitude journal, where they have to write 5 things they are grateful for each day, I'd even encourage that it be 5 things that OTHER kids might NOT have (bike, skating lessons, TV, etc) Try to get them to start thinking of intangibles they are grateful for (Mom who loves me, Dad's job, sunshine). Explain what "entitlement" means and that they AREN'T entitled to anything but food, shelter, and basic clothing. I reminded my older 2 frequently that "life, liberty, and the pursuit of happiness" was reserved for the tax-paying adults, thank you.

They are now tax-paying adults and are enjoying their life, liberty, and pursuit WITH the realization that it's a lot of hard work and money does not grow on trees.
One more thing: I do like the idea of a garage sale--their toys included--but I think at least part of the money raised should offset the cost of the trip beyond souvenir spending money. (Maybe snacks, and definitely a gift for you and dad!)
[Total side note: Once, en route to our family vacation, me and my little brother were whining about being hot and tired and hungry. My father warned us to behave ourselves or he'd turn right around and drive back home. At this point we were several hours into our journey and less than an hour from our destination, so of course we didn't believe him and acted up again. Wouldn't you know it--he turned right around and drove all the way back home. We never did it again. Only as an adult did I realize how tough that must have been for both my father and mother--and I later learned they fought about the decision for weeks afterwards behind closed doors--but it was a very valuable lesson for me.]