California enacted a child safety seat law that took effect on January 1, 2017.   All ESTA passenger vehicles fixed with seat belts require the use of seat belts for the driver and all passengers.  This includes all dial-a-ride passengers.  The law (VC 27318) requires the following

1) The driver, before departure of a bus carrying passengers, to inform the passengers to buckle up.  The vehicle to have signs informing the passengers of the requirement.
2) A sign will be affixed to the interior of the vehicles to remind passengers of the law.  If an adult does not have the proper child securement device for a child, the child may not ride the ESTA vehicle. Again, this applies only to vehicles fixed with seat belts.

These are the laws regarding children:

Children Under Eight Years Old

California law requires that all children under the age of eight be secured:
• in an “appropriate” child passenger restraint system that meets federal standards
• in a seat that is not in the front next to the driver.
The appropriateness of a car seat usually depends on the age and size of the child.  But generally, it’s going to be one of three types: rear-facing, forward-facing, or a booster seat.  However, for children who are 4’ 9” or taller, the law requires only a seatbelt. (Read about the different types of safety seats and restraints.)
(Cal. Veh. Code §§ 27360(a), 27363(d) (2017).)

Children Under Two Years Old
• On public transit, children under two years old may be held by a parent or guardian.

Children Between Eight and 16 Years Old
With children who are at least eight but less than 16 years old, drivers are responsible for ensuring the child is secured in an appropriate child seat or seatbelt.  For many kids in this age range, a seatbelt—rather than a car seat—is the appropriate type of restraint. (For more information on choosing the right car seat, go to the National Highway Traffic Safety Administration (NHTSA) website.)
(Cal. Veh. Code § 27360.5 (2017).)

Exceptions
A court can make an exception to any of the child safety restraint laws when application of the law would be impractical because of a child’s:
• physical unfitness
• medical condition, or size

And generally, a driver is excused from placing a child who is under eight years old in the back seat (see requirement above) when:
• there is no back seat
• the rear seats are side-facing jump seats
• the back seats are rear facing
• the child seat can’t be installed properly in the back seat
• all the rear seats are occupied by children under the age of eight, or
• medical reasons require that the child ride in a front seat.
A rear-facing car seat can’t, however, be put in the front seat when there are active front passenger airbags. (Cal. Veh. Code § 27363 (2017).)