hopefully a lawyer will address this as well but here’s what I would do:
-have the dentist/orthodontist write a letter - preferably directly to your husband, which recommends that your daughter needs braces, get a copy of this as well.
-send your husband a copy of your separation agreement with the portion about orthodontic care highlighted
What portion of the braces are covered by insurance? Who has the dental insurance? Many dentists offices will be open to some sort of payment plan if the cost is an issue for one or both of you.
If this does not get him to pay then you will have to take him to court by filing a “Motion to Show Cause” so he has to explain why he is violating the terms of your agreement. He will not have much to stand on if your dentist clearly shows that your daughter is in need of braces and orthodontic care is specified in your agreement.