Child support does support children and their activities, but that doesn't mean it covers everything a child needs. There are circumstances where parents may want to pitch in extra to support a child's new hobby or extracurricular activities. A problem some families have is that when one person pays support, that person might think that is the only monetary obligation. Realistically, that money only evens out the child's living conditions; it doesn't take the place of all related costs or situations.
Things like sports signups or new club fees are extras that parents have to pay if they want their children to participate. Sometimes children won't try to do these things because it could lead to a fight between their parents, but the truth is that parents need to be able to work together to handle extra monetary issues that come up.
One thing you could do is to work with your California attorney or a mediator to draft a plan for what happens if a child needs something child support doesn't cover, like a fee for a class trip or a new uniform for a baseball game. Financial arguments hurt children in the long run, so parents should focus on working through the financial challenges of raising a child, even if they aren't raising their child in the same home together.
Focus on what your child needs and work with the other parent. It may be that you don't want your child to do certain things that your ex-spouse does, so your ex may foot the bill, just as you might if you have a particular activity you want your child to participate in. Working out the details before arguments arise helps you and your child by removing financial stresses in daily life.
Source: Brentwood Patch, "Parenting and Child-Related Money Issues After Divorce," Sep. 07, 2016