BeckyW has the best answer for "when may my child sit in the vehicle's seat belt system" -- the 5-Step Test (see www.carseat.org). Although California law (27360 VC) states minimum 6 years or 6 pounds for a child to be out of a car seat (including boosters), remember, that is a minimum. Note that California law also states you may take your child out of school when they are 16. Just because it's the legal minimum, doesn't make it the best choice.
Here are the basic "rules" based on safety, not the law.
Rear facing (RF) is safer as the shell of the seat absorbs a great amount of crash impact. Keep your child RF as long as possible (2+ years). New car seats are now available with a RF limit up to 45 pounds! Don't worry about the legs up against the seat back. "Legs and feet heal, brains and spines do not." If your infant seat has a limit of 20 to 30 pounds and your child has maxed it out, change to a convertible seat and place it RF in the car.
Use forward facing (FF) with harness to the maximum size and weight allowed by the car seat manufactuer. FF harness seats are now availalable to 80 pounds. Only after maxing out the RF and FF harness seats, should a child be placed into a booster.
The child no longer needs a booster when he or she passes the 5-Step Test "in that vehicle."
Remember, children do not "graduate" from RF to FF to booster. Each step is a demotion in safety.
Finally, please make sure your car seats are installed correctly. Don't guess! Get help if you are not certain!
-- M. Cohen
Passionate Child Passenger Safety Instructor
Burbank CA
cps -at - marccohen -dot- net
Loving Step-Father of Melissa Monique 1980-2002
Loving Grandfather of Alec Jaye 2000-2002
"Again this year, Alec and his mommy won't be visiting grandma and grandpa for the holidays"