Half of children's car booster seats can't ensure a proper fit with all safety belts, an insurance industry-funded safety group says in a report out today.

The Insurance Institute for Highway Safety said six were so bad that it recommended parents avoid them.

Booster seats, which are recommended for children who have outgrown forward-facing child seats, are designed to raise kids up so adult-size safety belts fit properly.

"Not all boosters are doing that well," says Anne McCartt, the institute's research chief.

Children ages 4-8 in booster seats are 45% less likely to be injured in a crash than those using only seat belts.

Booster seats were rated based on how well they fit the roughly 20 million 4- to 8-year-olds with the lap and shoulder belts in a wide range of vehicles.

IIHS says its ratings are important because it's impossible to tell which booster seats are better just by comparing prices or features.