I'm with Gen X on this, if you've told them the trip was in jeopardy if their behaviour didn't change then you're on the hook to cancel. Consequences should be clearly defined and once defined followed through on.
If you don't follow through then their no consequences attitude is no-one's fault but yours.
My apologies if my first reply sounded trite, I wasn't trying to be. Your initial post didn't indicate you'd already set the groundwork. My next bit of advice is to not use things you don't want to lose yourself as the consequential loss for them. My son is an avid reader and I knew early on that taking his before-bed reading time away would be a punishment that would hit home. He pushed back once and lost his reading.The second time, he lost his reading but kept doing what I was punishing him for... So he lost a second night's reading. When he realized it was cumulative, he never pushed it again! I've been lucky that he learned early on that Daddy doesn't back down.
Hard as it may be, you need to crack down. No prevaricating, if you say something, mean it every time. From the way you've phrased your comments, it looks like you use less than clear terminology. Start saying "If X, then Y" and then if X happens, make sure Y follows quickly and it's clearly defined that Y is happening because X happened. If you allow wiggle-room, your kids will run rough-shod over you in a heartbeat.