The organisations say older children who have outgrown front-facing car seats should travel in booster seats until the lap-shoulder belt fits them. Booster seats help position adult seat belts properly on children's smaller frames.
Children usually graduate from a booster seat when their height reaches 4ft 9ins.
Children younger than 13 should ride in the back seat, the guidelines say.
The advice may seem extreme to some parents, who may imagine trouble convincing older schoolchildren -- as old as 12 -- to use booster seats.
But it's based on evidence from crashes. For older children, poorly fitting seat-belts can cause abdominal and spine injuries in a crash.
One-year-olds are five times less likely to be injured in a crash if they are in a rear-facing car seat than a forward-facing seat, according to a 2007 analysis of five years of US crash data.
Put another way, an estimated 1,000 children injured in forward-facing seats over 15 years might not have been hurt if they had been in a car seat facing the back, said Dr Dennis Durbin, lead author of the recommendations and a pediatric physician.
Toddlers have relatively large heads and small necks. In a front-facing car seat, the force of a crash can jerk the child's head causing spinal cord injuries.
Car seats have recommended weights printed on them. If a one-year-old exceeds the recommendation of an infant seat, parents should switch to a different rear-facing car seat that accommodates the heavier weight until they turn two, the pediatricians group says.
Luckily for parents, most car- seat makers have increased the amount of weight the seats can hold.
This year, about half of infant rear-facing seats accommodate up to 30 pounds, Dr Durbin said. Ten years ago, rear-facing car seats topped out at children weighing 22 pounds.
"The good news is it's likely parents currently have a car- seat that will accommodate the change," Dr Durbin said.