First, here is a link to how-to information on filing a small claims action in California:
https://selfhelp.courts.ca.gov/small-claims-california
Knowing how transactions are submitted and recorded by both Chumba and by the bank will be necessary. Just because you authorize four payments of $20 on, say, Monday, Chumba may not submit those payments to the bank on Monday. Chumba instead can wait for Tuesday, Wednesday, Thursday and Friday’s authorized payments and submit them all at once. Chumba and the bank’s transaction histories could, therefore, differ in that way.
I think you will struggle trying to show unauthorized payments were made from your account. You could subpoena records from Chumba, and you should be able to get a printout of your bank transactions from your bank that cover the month(s) you played games on Chumba. I
suspect these payment transaction records will show that Chumba’s submissions equal the bank’s withdrawals. And you don’t appear to have the necessary evidence to show that some of the submissions from Chumba were unauthorized.
You cannot have an attorney in small claims courts but you can consult with an attorney prior to filing suit. I suggest you have an attorney in your area review this matter with you.