During the winter months, a parent or caregiver's instinct is to bundle up a child to put the child in the car. However, putting a child wearing a bulky winter coat into a car seat can be dangerous.
Most car seats will not work properly if the child is wearing anything thicker than a sweatshirt. If you have a car crash, bulky coats, or other sleeping bag-like materials, will compress. As a result, the harness straps will become too loose to hold the child in tightly and help the child come to a gentle stop. The child could even slip out of the restraint.
Another reason you should remove the child's coat before putting him or her in a car seat is because, as the heater warms the car up, the child, unable to remove the coat, could overheat. Amie Durocher "Winter Coats and Car Seats" saferide4kids.com (Dec. 20, 2018).