If you truly can't afford your vacation so much that you're stressing out and yelling at your kids to HAVE FUN when they're tired becuase you know you'll never get there again and can't afford it even this time...then it's too expensive. I don't think small children can make themselves appreciate "once in a life time" vacations any more than "regular" vacations ... they are going to be themselves, no matter if what you're doing is cheap or expensive. (If they always take a nap from 2 to 4, they are going to need a nap from 2-4, even if you're in the MK.)
I'll express a different viewpoint from the posts above me. For all you parents who run up credit card bills and sacrifice a great deal to take your kids on Disney or any other vacations, I'll say this: Maybe you should re-think this. My parents were children of the Depression. They were both from large, poor families and literally never had a vacation growing up. Becuase of that, we always went on vacation. Sure, we drove and stayed at Motel 6s and cooked in the room, but we always went on a vacation. BUT, my parents rented a house all while I was growing up...they couldn't afford buy a house until I (the youngest) was 20 years old because they didn't have the money for a down payment earlier. When I look back on it, I think it was foolish to take a vacation every year...it would have been much wiser to have skipped a few years to save money for a down payment on a house. They would be much better off in their retirement if they would have skipped a few vacations and bought a house and invested money.
I know my parents were trying to give us the nice childhood they never had, but I don't know that it was the best decision.