Child Support Services only has the riht to order the payment of certain things. Usually, in a dissolution in family court the Parties agree to split the expense of the children's activities that both Parties agree to and that one Party has not put them in to interfere with the other Parties' visitation. If necessary, you should put this issue before the family court since shared expenses is not an issue for Child Support Service.
Some courts hold that the only items that can be added to child support are those specifically listed in California Family Code section 4062; to wit, (1) Child care costs related to employment or to reasonably necessary education or training for employment skills; (2) The reasonable uninsured health care costs for the children; (3) Costs related to the educational or other special needs of the children; and (4) Travel expenses for visitation.
If the judge hearing your case adheres to this (legitimate) reading of the statutes, he/she may deny your request. BUT, that does not mean you shouldn't make the request. Many judges add items such as reasonable extracurricular expenses, therapy costs, etc. Some who won't add the items unless they are by agreement will turn the issue on the other parent, asking them squarely if that activity is something they want to agree to. In a courtroom setting, most parents want to appear cooperative, so a question might get a "yes" when asked by a judge, even if it got a "no" when you asked it.
Even if your judge turns you down, it won't anger the judge that you made the request, especially if it is a long-standing activity for the child. I would suggest that this falls under the "doesn't hurt to ask category." I do agree with the previous answer as well, if your case is being handled by the Department of Child Support Services, don't ask their attorney to bring it up (he/she does not represent you and making this request could be seen as advocacy for one side over the other). Make the request yourself.