ROYAL OAK, Mich. -

In this bitter cold, you want to keep your kids bundled up and get them in and out of their car seats as quickly as possible. But safety experts say if you're buckling them in with a bulky coat, your children may not be as safe as you think.

"Bulky coats or snowsuits are not advised in the car seat," said Erica Surman, a certified child passenger safety technician at Beaumont Children's Hospital. "The reason is you want to get the car seat straps as tight as possible to the child, so that it keeps them secure in the car."

Although most parents we asked did't seem to be aware of the "no coats" rule, Surman said it's actually written in the manual for most car seats.

"I never, never heard of this," said Zachary Walsh, a Royal Oak father of two, with another baby on the way.

Walsh has years of experience buckling up 5-year-old Beckett and 3-year-old Emmett, but never knew coats could be a concern.

"It's cold outside, you're trying to protect your kids by bundling them up," said Walsh.

He agreed to let Emmett demonstrate the problem.

First, Surman put Emmett in his car seat wearing his winter coat. She did something called the "pinch test" to make sure there isn't any slack in the straps.

"We check the 'pinch test' over at the shoulder," said Surman. "We shouldn't be able to pinch here."

Next, Surman took off Emmett's bulky coat, leaving him in a thin fleece instead. She put him back in the car seat, without readjusting the straps.

So how much space was his coat taking up? A lot. Enough to fit a teddy bear between Emmett and the straps.

"There's quite a bit of slack. What we thought was tight is actually not tight at all," said Surman.  "There's so much slack, in fact, his little buddy Beau the bear could ride along with him."

Experts say in a crash, your child's puffy winter coat will compress, making the car seat straps far too loose.

"Just like when you push down on a down comforter and you can flatten it, or when you use a vacuum seal, you can see that you're taking out all the air," said Surman.  "In a bad accident, all that momentum will compress (the coat) into just something very thin."

As a result, your child could suffer serious injuries or even be ejected from the car.

"It makes a significant difference," said Dr. Kelly Levasseur, a Beaumont pediatric emergency medicine physician.  "Even one inch of play in their straps could lead to significant damage, especially in the cervical spine area. That means neck injury, severe neck injury, and can lead to paralysis of arms or legs."

Levasseur said having your child properly restrained in their car seat could be the difference between them walking away from a crash unhurt or suffering severe injuries.

"It's really important to always restrain your child properly, no matter how long of the drive," said Levasseur.

The demonstration was eye-opening for Emmett's dad.

"It was impressive," said Walsh. "Sticking the bear in there and really seeing the difference that that coat makes, it's clear that just that inch or couple inches can really make a big difference."

Safety experts say this isn't just a risk for babies and toddlers. Older kids in booster seats should also not wear heavy coats.

So how can you keep your kids warm without putting them at risk?

"We usually recommend keeping their hats and their gloves on and something really warm like a fleece," said Surman.

Surman also suggested turning your child's coat around backwards and putting it on over the car seat straps or just covering them with a blanket instead.

Surman is also a mom, and she practices what she preaches with her 4 year-old daughter.

"Oh yes, she won't even wear a coat in the car," said Surman.

"It's one of those things that you hear that, 'Oh my gosh, now I'm going to have to maneuver coats and all that,' but if that's what you have to do to keep them safe, that's what you have to do," Walsh said.

Studies find up to 85 percent of car seats are not properly installed. Safety City U.S.A. and Beaumont Children's Hospital have teamed up with Mike Savoie Chevrolet to offer free car seat safety seat checks for local families. The checks will be held at Mike Savoie Chevrolet at 1900 W. Maple Rd. in Troy on the second Thursday of each month.

For more information, click here.

Click here for coats that are considered safe to wear with a car seat.

Click here for more options in safe coat wear for kids.