Can my ex force me to pay for half of braces without my consent

I am divorced, with three children from my previous marriage. We had a consultation and where it was recommended that two of the three children have braces. I explained to my ex that I do not have the money to pay for both of them at the same time, And that I would like to do one at a time so I can afford it. She refuses to give at all and is going to move forward. My question is do I have to pay for both of them or either of them without consenting? Can she force me to without my permission? I live in New York State.