In places so cold, parents are forced to heavily dress up their kids in order to keep them warm. However, this may make the car seat unable to do what it was intended to. In case of a crash, the harness will make sure that your kid is held in place to avoid him from being injured. The danger of excess heavy padding between your kid and the straps is that if a crash occurs, the padding compresses hence loosening leading to your child being hurt or be thrown out. If your car hits on something, the weight of all people in it goes up and in this case, your child who may be putting on the flattened jacket will not be safe since the straps will be very loose to hold him. Having a jacket or a blanket while the straps are on, will not keep your kid safe from a crash.
It is advisable that you test first whether your child will be safe with a jacket on in order to know if it is the best cloth for him to put on. Dress up your kid with the winter clothes and then put him on the best car seat while tightening it. Remove him without loosening the straps and then place him back without the heavy clothes. If it leaves a space then it means the coat is too heavy and may not be a good idea to put on. A good way to know your kid is safe with clothes on is if the straps snugly fit the same way if the clothing is on or not.
As a parent, you kid putting on that heavy winter clothing may be the safer way to keep him warm. Although that may be the case, such heavy clothing prevents the straps from fitting well, therefore, changing his position which may be dangerous in case a crash occurs as your kid may be ejected and can be severy injured in the process. Having a booster seat is the best way to boost your child up in order for the seat belt to perfectly fit. The reason why a heavy coat or clothing is discouraged is that the lap belt will go up on his abdomen, therefore, getting him much exposed to injuries in case of a crash.
If it is winter time and the only way to keep your kid safe is by having a winter jacket then make sure to tighten the harness system to fit well. The best and assured way of keeping your child warm and safe is by putting on thin and warm layers. A fleece clothing in such weather will be perfect since it will not add any extra layer under the seat belt, thus, the safest clothing to put on. A blanket can be covered on top of straps and not under. In very cold season, a puffy coat can be worn by your kid on top of the straps with the back coming in front since it is also important that your kid is kept warm. Also, make sure to keep on checking if your kid has warmed up and if so remove the extra clothing on top to prevent overheating which is dangerous to your baby.