Re: why doesn't a baby shiver when it is cold?

Shivering is a means of thermoregulation (controlling body temperature) 
which can generate heat by muscle activity. Infants are thought not to be 
able to do this due to poor development of muscle tissue. Instead, infants 
use nonshivering thermogenesis (heat production) as a means of generating 
heat. This is accomplished in infants by basically burning fat using their 
storage of brown adipose(fat) tissue(a mammalian heat-producing tissue 
occurring especially in human newborns and in hibernators ) which is found 
in infants up to one year of age and in some adults acclimated to the cold.

hope this helps,
gabriel vargas, md/phd

References:
Human Adaptability:An introduction to Ecological Anthropology
Emilio F. Moran