Dress kids in layers. If one layer gets wet, the child can remove it to keep moisture away from the skin.
Use fleece and wool garments instead of cotton clothes, which will not provide insulation if they get wet.
Warm boots and mittens are essential because extremities are most prone to frostbite.
Make sure kids cover their ears and nose and wear a hat to prevent significant heat loss from the head.
Children should come inside often to limit their exposure to the cold, and they should change out of wet clothing immediately.