@becausehefirstlovedus Try ‘I am asking for your reasonable consent to take x and y out of school on abc dates. We have an opportunity to attend an extended family reunion (or whatever you have planned) and for xyz reasons, this timing is best for us to take advantage of this opportunity.
I have spoken with the children’s teachers and they have agreed to provide me with the schoolwork the children will be missing, which we will complete before their return to class. The teachers confirmed the kids are not missing any important tests or events.
Thank you in advance for your cooperation.’
If ex still says no, try again a few more times for different events, documenting it all and then take it to court to ask for clarification on the language in your plan. Or, if you have enough documentation, take it to a mediator now to get clarification and have it legally changed.