You can go online and get the CA child support worksheet or call the clerk of the court in your county and ask them. It's hard to know how much child support would be assigned to him without knowing exactly how much he makes if he does overtime. The court would have to see his w-2s.
In any case, you'd be more secure if you had a court mandated child support order in that you would not be relying on him to send the money each month. If he's not reliable about visitation with the boys then he probably can't be counted on to remain reliable with the child support check.
Does he provide health insurance for the children? In CA you can get additional payments for child care expenses, such as baby sitters or day care, if you are working.