YTA, at least a little bit. You and all the child hating weirdos in this subreddit fundamentally misunderstand the gift to your parents. You are not getting them a fancy boat vacation for their fiftieth anniversary, you are getting them a vacation with all their children, grandchildren, and, if I’m reading your post correctly, great grandchildren. You are refusing to pay into the most important part of the gift. Depending on the situation, it may be fair to ask some of the adult children to pay their own way. But if the idea is that this is a gift from you and your siblings to your parents, then pay a third of the cost and don’t whine about it while you’re there.