More facts are needed to understand your question. It is highly unusual for someone to be charged for working. California and Federal law generally require payment of minimum wage to the employees.
You can file a FREE wage complaint on line with the Division of Labor Standard Enforcement.
Your facts are unclear as to what "charge money" means. I am involved with a not for profit, the Boy Scouts, and participation costs me money every year on the troop level, the district level and the council level. Further it is hard to imagine that you did not consent to what you are doing.
The above is general legal and business analysis. It is not "legal advice" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.