Something to be aware of for next year. Earlier this month Governor Brown signed into law SB 929 to change child booster seat requirements and align them with National Highway Transportation Safety Administration standards effective January 1, 2012.
Current law requires children in the State of California under age 6 or under 60 pounds to be seated in a child restraint seat (e.g. booster seat).
The new law changes this requirement to children under 8 years of age, unless they are 4' 9" or taller.
As a father of kids in this age range, I do think this new law makes sense. The lap belt is supposed to cross the hip area, not lie across the tummy, while the shoulder belt should lie across the chest, not hike up the neck. We've experimented and without the booster seats my kids are simply not fully protected and the belt could do some major damage to their small bodies if not situated correctly.
This law is interesting in that it takes away the weight parameter and replaces it with height. Your kid is "off the hook" if they are 4' 9" or taller before the age of 8. Jersey Shore "celebrity" Snooki Polizzi is 4' 9" tall at the age of 23. If she were just slightly shorter perhaps it would be in her interest to use a booster seat in her car.
I digress. It is interesting how laws vary state by state on this issue. The majority of states require kids 7 and under to sit in booster seats, yet it is currently perfectly legal for a 4 year old to use an adult seat belt in the state of Florida, while Arizona and South Dakota allows 5 year olds to wear adult seat belts. At the other extreme, kids under age 9 in Wyoming all must be in kid seats.
Check out www.iihs.org/laws/mapchildrestraintagerequirements.aspx for well organized information regarding child seat belt laws by state.