Unfortunately there are not enough facts provided to give the best answer, namely whether there is currently a child custody agreement/separation agreement/court order in place, which often has a section specifically pertaining to how the parties are to agree regarding extra curricular activities before such expenses are incurred and thus forced upon the other to pay all or even half. And/or whether the current child support amount of $600 is by a child support agreement/separation agreement/court order (sometimes child support is in the same document as child custody, but not always). But generally speaking, an agreement can only be changed upon mutual consent of both parties and a court order can only be modified by filing a motion to the court and proving a "substantial change of circumstances" which is not always as easy as the claimant thinks and certainly in this scenario, would not seem to be a critical need for the child or an otherwise significant change worth disturbing a current order. You should contact an attorney to review your current documents and how/if they might be changed and if so, how you can defend against that.
Answered on May 28th, 2013 at 9:56 PM