Though it is often under-appreciated, the sense of warmth is an extremely important sense young children are developing. This sense is not yet fully developed until around third grade. In addition to feeling physically warm and comfortable, having warmth also helps develop the overall sense of well being that allows children to feel safe and secure and able to learn both academic and social-emotional skills.
Before around third grade, children are not able to discern whether they are warm enough. Though we want our children to learn to trust their bodies and to honor their decisions about their bodies, we can not yet rely on them to tell us whether they are warm enough at this age. They are just not developmentally able to know when their body is too cold.
One reliable way to know if a child is physically warm enough is to feel their hands. If their hands are cold to the touch, the child needs to add another layer. Children often resist wearing coats, hats, mittens, and other outerwear for many reasons, however, their safety and development rely on us to keep them warm.
If you are interested in learning more about the sense of warmth and its impact in early childhood and beyond, here are a few articles for further reading: