Are you saying that the parenting plan you have with the mother grants the mother sole decision-making authority regarding non-emergency medical expenses? In my experience, braces are considered a non-emergency medical expense. Have the braces already been installed or is this just something the mother says is going to happen? If it's already happened and the the mother has sole decision-making authority, I believe she can bind you to such a thing as far as your income proportional share. If it hasn't yet happened, you may be able to file a motion with the Court objecting to the mother's proposal and letting the Court make the decision. If you two have joint decision-making authority regarding non-emergency medical, then you may need to go to dispute resolution regarding this issue if it says so in your parenting plan. I recommend you secure an in depth consultation with an experienced custody attorney in your area as soon as possible to go over your Parenting Plan and Order of Child Support to determine what the best road for you would be.
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A good case on point is: MARRIAGE OF LESLIE, 112 Wn.2d 612, 772 P.2d 1013 (1989). You can see it through www.mrsc.org. There, a divorced spouse had obtained a declaratory order requiring her former spouse to pay for a child's orthodontic expenses. The Court of Appeals and Supreme Court upheld the order.
Calling orthodontic expenses "medical" or "cosmetic" isn't really the right question. The question(s) may include the child's need and the ex-spouse's ability to pay. "Child support" is an entirely different question which calculates payment depending on the age of the child and the available resources of the parents.
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