Y don't kids on the street bundle up

Babies and children have more brown adipose tissue, i.e. brown fat - than adults. This specialized fat is a highly efficient heat producer, warming the body's blood, especially around the neck and shoulders. 

Children have a naturally higher metabolic rate than adults. Their bodies generate more heat from the start. Children are much more active than adults, and physical movement generates body heat.

Children have higher skin temperatures and their bodies recover blood flow and temperature more quickly after cold exposure.