Non-custodial Parent Not Purchasing Plane Tickets For Children To Visit

Looking to see what repercussions I might face, if any, in this situation.
I moved about 1200 miles away from my home state last year with my children, their father lives in our home state still. We amended and agreed to a new parenting plan when I moved that gives him 6.5 weeks of visitation per year, the kids take unaccompanied minor flights, and we take turns paying for them.
I paid for their November flights, he (well, his gfs mom) paid for their December flights, I paid for their March flights, and he is supposed to be paying for their June flights.
The kids are supposed to leave on June 2nd and stay with him for three weeks, he hasn’t purchased the tickets yet. I understand there is still time, but if he doesn’t, is there any action that he can take against me for him not being able to see them?
The parenting plan clearly lays out the visitation times and dates, and who is responsible for purchasing tickets and when. I have held up my end of the agreement, he hasn’t. I offered to let him purchase one way tickets this time and I would finance the trip back myself, he still hasn’t purchased the tickets.
Legally speaking, if he tried to take this back to court, would not sending them on my own dime when he was responsible for travel arrangements this time look badly for me?
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