Kindergarten orientation - ok to miss?

My ex husband is taking the kids to the beach with family this Saturday for a week. Our youngest starts Kindergarten the Monday after. Orientation is the Thursday they're gone. I think it's really important that she be there (they'd have to come home early) but he thinks it's irrelevant and that their vacation takes priority. Am I overreacting (I mean I am 36 weeks pregnant lol) saying she needs to be back for orientation or she can't go on the trip or is it really not that important and it's ok to skip kindergarten orientation?