It may seem like a heavy winter coat is the best thing for him when it’s cold. But when it comes to riding in a car seat, a thick coat or jacket can be lethal. Puffy winter jackets trap air, keeping car seat straps a dangerous three to five inches from his body, according to kidsandcars.org. That leaves your child susceptible to being flung from the seat in the event of a crash. Instead of putting him in a big coat, try multiple thin, light layers. Make sure the car seat harness is snug against your kid by grabbing the straps near his shoulders and pulling upwards. There should only be enough room for one of your fingers between the strap and his collarbone.