According to IIHS (
Child Restraint Laws), the California law is that children 5 years and younger or less than 60 pounds must be in the rear seat. Many other states express no preference for rear seat. There are exceptions in CA, however.
In California, children weighing more than 40 pounds may be belted without a booster seat if they are seated in the rear seat of a vehicle not equipped with lap/shoulder belts.
The California rear seat requirement does not apply if: there is no rear seat; the rear seats are side-facing jump seats; the rear seats are rear-facing seats; the child passenger restraint system cannot be installed properly in the rear seat; all rear seats are already occupied by children under 12 years; or medical reasons necessitate that the child not ride in the rear seat. A child may not ride in the front seat of a motor vehicle with an active pasenger airbag if the child is under 1 year of age, or weighs less than 20 pounds or is riding in a rear-facing child restraint system.