Are your children's coats putting them in danger in their car seats?

It's a matter of inches, but experts warn in a serious crash, it could be the difference between walking away unhurt or suffering severe injuries.

Zachary Walsh had no idea bulky coats in the car could put his children in danger. Yet there it is, in print, right in the car seat manual. Sometimes seeing is believing, says Seattle Children's Brittany Blue.

In a crash, puffy coats will compress, making the car seat straps far too loose. As a result, children could suffer serious injuries or even be ejected from the car.

The solution is a light-weight jacket. Add a blanket if it's chilly. The same goes for infants, plus one more step.

Do the arm pit test. Buckle them into the harness system and just raise the arm up, Blue said. If the chest clip is hitting them at the armpit level, it's perpendicular. Then that is generally a good fit.

Be sure to check the manufacturer's date on the car seat. If it's more than six years old, toss it. The same goes if it has been in a crash.

It's also critical to make sure your car seat is installed properly. Up to 70 percent are not.