I agree with the previous poster about setting rules and following through. My friend told her two boys that one fight and they would leave the park and go back to the hotel. Of course on the first day, they started in on each other, and even though she was MISERABLE about it, she walked them right out of the park and took them back to the hotel. They cried and were shocked that mom was serious. Two hours later she told them one more chance, but if they started up again they would leave again and spend the whole day in the hotel. Those kids were angels the rest of the time because now they KNEW she wasn't bluffing. One day of misery was worth it for the five good ones that followed....
No flames please, but you might try a really fun reward system. No fighting/squabbling will earn them something while on vacation...call it what you want...bribery works for me. To be honest, I'm not above it, and I say whatever will get you happy memories of your family vacation is worth considering.
Good luck....
Mel