It probably varies by state, but I can't imagine most states consider contracting an appropriate occupation for children. You might double check on CA law, but per this Child Labor Pamphlet
children under 16 can't be a "student learner or apprentice" in Occupations involved in the operation of power-driven woodworking machines and Occupations involved in the operations of circular saws, band saws, and guillotine shears .
Not sure what type of work they were doing, but it might not be legal from him to take his son on help on job sites.
Not that you should threaten the contractor, maybe just point out that it would be unfortunate if the state was notified.