When buckling a child in their car seat, take off their winter coats. If their coat is left on, the harness straps do not get tight enough.
Try to pinch a horizontal fold in one of the straps above the chest clip. If you can pinch it and grab it, it’s too loose. If you can sort of pinch it but you can’t grab hold, it’s tight enough. And there should be no slack in the hip area.
After the child is buckled in, you can put their coat over top of them or use a blanket to keep them warm. The link below is a news story so you can see firsthand the dangerous effects of wearing a winter coat in a car seat.