Legal Question in Family Law in Texas

If he won't agree, the court won't adjust the summer schedule to accommodate your plans.

If he agrees, and then you purchase the tickets, and then he changes his mind, you could sue him for the cost of the tickets and your other financial damages, including reasonable attorney's fees. You would need to get his agreement in writing so that you can prove there was an agreement. You'd need to make the agreement state that "but for his agreement to modify the summer schedule", you would not have purchased the tickets.

It doesn't have to be suspiciously formal. You just need to cover your bases. For example, you could ask him, "I have not purchased tickets for the children yet and I won't do so until we have an agreement to modify next summer's schedule as I outlined above. Please let me know whether you agree to that one-time modification so that I can know I'm safe to purchase tickets." If he responded saying that he agrees, then you probably have an enforceable contract.

However, if you purchased the tickets before he agreed or without a written agreement (and email/text is fine for "written"), then it would be hard for you to claim that you relied on his promise when you purchased the tickets. If you buy the tickets first, it will look like you assumed the risk and it just didn't work out.