This is the California law as written, found on http://ohv.parks.ca.gov/?page_id=27288
38603. (a) A person operating a recreational off-highway vehicle with a model year of 2014 or later shall not allow a passenger to occupy a separate seat location not designed and provided by the manufacturer for a passenger.
(b) Seats that are installed in a separate seat location not designed and provided by the manufacturer for a passenger in vehicles with model year of 2013 or earlier may be occupied by a passenger if the occupant of the seat is fully contained inside of the vehicle's rollover protection structure at all times while the vehicle is being operated.
Regarding aftermarket seating:
"Any manufacturer provided seat can be replaced with an aftermarket seat; however, for 2014 model year and later models, seats cannot be added in areas of the vehicle that were not designed as seating areas by the manufacturer"
Child car seats are not specifically addressed in the law so ????