Six- and 7-year-olds who had "graduated" from their booster seat to a passenger seat will find themselves back in the saddle come Sunday, thanks to a new law designed to increase child safety in California.

California state law currently requires parents to keep their kids in booster seats until they reach the age of 6 or weigh at least 60 pounds. The new law does away with the weight limit and requires children to stay in booster seats until they hit 8 years old or 4 feet, 9 inches tall - a height that very few children will reach before that age.

While the new requirement is likely to provoke a collective shriek from children throughout the state, child safety experts say the change is for their own good. Studies show booster seat use for children ages 4 through 7 decreases the risk of injury by nearly 60 percent compared with seat belt use alone.

"The No. 1 killer of kids is car crashes. I just want all the families to know how important child restraints are and how they can protect their kids," said Livermore police Officer Traci Rebiejo, who testified on behalf of law enforcement in support of the legislation, which was authored by Sen. Noreen Evans, D-Santa Rosa.

Booster seats are necessary, experts say, because seat belts are designed for adults and the boost puts children in position to be properly restrained. Research has shown that small passengers whose seat belts fit improperly were at an increased risk for serious injury or death in car accidents.

Vetoed twice before

Although similar legislation was vetoed twice over the past six years by former Gov. Arnold Schwarzenegger, who objected to imposing a new law telling parents how to keep their kids safe, more than 30 states have surpassed California's current requirements. Gov. Jerry Brown signed the new standards into law Oct. 4.

Mountain View parent Matthew Kagle said his 7-year-old son, Simon, who had been out of his booster seat for a year and a half, didn't take it so well when he learned he would have to go back in it.

Strong disapproval

"You would think I had told him he had to have an extra injection. He wanted to write a letter to Jerry Brown to express his disapproval, but he doesn't quite have the wherewithal to do that yet," said Kagle, who also has a 4-year-old son who just graduated from a car seat to a booster.

Kagle said his older son, who is in the 98th percentile in height, is hoping he hits the 4-foot, 9-inch limit before he turns 8 in June. Kagle says his kid is a tight fit in the seat: "It's like having Andre the Giant in a kindergarten seat back there."

Carol Powers, child passenger safety program coordinator at John Muir Medical Center in Walnut Creek, said some children should actually be kept in booster seats after the age of 8.

"It isn't about how old you are; it's about how the belt fits," Powers said. "The booster seat just makes the seat belt fit on the right parts of the body."

Parents should make sure that the lap belt is low on the hips, touching the upper thighs, and that the shoulder belt crosses the chest, but not the face or neck, Powers said. Children should never be allowed to push the shoulder belt behind them.

A lesser-known aspect of the new law makes parents responsible for their child passenger's safety until they reach age 16. Parents or drivers can be fined at least $475 and get a point on their driving records for each child under 16 who is not properly secured.

Tadashi Nakadegawa's three children - 7-year-old twin girls and a 10-year-old son - are all under the height limit. While the girls technically "aged out" of the seats at 6, Nakadegawa has kept them in boosters because of their size.

"For me, anything that would have you keep your kids in the safest restraints possible is good," said Nakadegawa, of Oakland.

He said he would prefer to keep all his kids in boosters as long as necessary, but he acknowledged it would be tough on his son, especially on field trips or with friends.

"If your kid is the only kid in a booster in the whole class, that's a hard thing to deal with. But I still prefer they use it," he said.

-- For more information about car seats, the new law or help in determining whether your child needs a booster seat, call your local health department or go to links.sfgate.com/ZLFR or links.sfgate.com/ZLFS.

About the law What is the change? Effective Sunday, all children under 8 or a certain height must be properly buckled in a car seat or booster in the backseat. Previously, the requirement applied to children up to the age of 6 or 60 pounds. Does that mean a car seat or booster? Typically, children outgrow their car seats at age 4 or 40 pounds, depending on the seat manufacturer's instructions. Now kids who move from car seats will have to remain in boosters until they turn 8 or reach 4 feet, 9 inches tall. So weight doesn't matter? Studies have shown that height is a better indicator of whether a child needs additional security. Seat belts are designed for adults and typically do not fit children, regardless of weight. How should the belt fit? The lap belt must ride low on the hips, touching the upper thighs, and the shoulder belt must cross the chest. Never use a booster with a lap belt only. Is there a penalty for not following the law? Yes. Parents or drivers face a minimum fine of $475 and a point on their driving record. Source: Safe Kids California.