It appears that children (pre-puberty) are not as well equiped as adults to deal with heat. As such, they are more prone to 'overheating'. There are several reasons for this: 1) their sweat glands have not matured and are not as efficient as their adult counterparts 2) they have a higher surface area to body mass ratio ans as a result they tend to warm up faster when exposed to environmental heat and 3) under stress and/or activity, their hormone system is not fully developed and, as such, their sweat glands are not receiving as many signals to perspire.
Finally, the chart at the right provides an indication of climatic zones and their relative safety with respect to activity and the risk of hyperthermia. Of course the higher the temperature and humidity, the greater the risk and need for precautions such as hydration and rest periods.
Temperatures over 27C (80F) are particularly troublesome as even low rates of humidity give rise to 'alert' or 'danger' zones. For more details on climate, heat stress in children and recommendations from the American Academy of Pediatrics click on sun icon.
Gomes LH et al Rev Paul Pediatr 2013