Ex wants the kids out of school early for a vacation but the kids don’t want to miss the last week.

My ex husband wants to pull the kids out of school a week early to go to their house in Mexico. I asked if they could push it back a week when they are actually on break because the kids don’t want to miss the fun things they do the last week of school, they are kindergarten and preschool but they love when the fire trucks come and they do water games at school the last week. He won’t move the trip because he wanted to do a weekend trip the next week with his girlfriend and his mom won’t move her hair appointment that week (she’s going on the trip as well) . Is it fair that I’m irritated that they want the kids to miss the last week of school but they as adults won’t move their hair appointment and weekend trip? I prioritize trips and appointments around my kids lives, I don’t move their stuff to work into my life but he always expects me to move their stuff to work into his schedule.. I’m very frustrated but he thinks I’m being unreasonable for asking to wait one more week. This trip would be around the end of may and then he could take them anytime during summer vacation. I’m trying to make everyone in this situation happy but someone’s going to end disappointed and I’ll be the bad guy because the final decision lands on me. Advice? Suggestions?