Kids don’t adapt to extreme temperatures as well as adults. They generate more metabolic heat per mass when active, have a reduced capacity to sweat, and their high surface area to body mass ratio means they absorb more heat from the environment.
When it’s over 95 degrees F, kids’ exercise tolerance is lower than adults. Humidity only adds to heat stress. Getting used to exercising in the heat takes 8 to 10 exposures of 30-45 minutes each. These can be done one per day or one every other day.
Kids must drink to replace fluids lost during exercise, but they don’t always feel the need to drink enough. Dehydration increases the risk of heat stress and illness.
Children learn what the live, and adults should teach proper health habits in hot and humid weather: stress acclimatization, appropriate clothing, fluids and adjusting activities based on heat and humidity.
A chart that plots the Heat Index from temperature and humidity into relative risk zones is here. The colored zones indicate when parents, kids, coaches and players should exercise caution.