Under California Vehicle Code Sections 27360–27368, taxis are not exempt from child car seat laws. Children under 8 years old or under 4’9” tall must be secured in an appropriate car seat or booster seat regardless of the type of vehicle, including taxis. The law ensures the safety of young passengers and applies uniformly to all drivers.

Taxi drivers will refuse to take kids without proper car seats. But not to worry, For parents traveling in the Bay Area,

Airport Taxi Cab Services ensures compliance with these laws by offering car seats and booster seats, making it a safer and more convenient option compared to other transportation services. 

Official Government Source, Not just a forum talk: Source