Children are more susceptible to heat stress than adults for a number of reasons:
Children don't have as much ability to sweat, to dispel heat
Children begin sweating only when their body reaches a higher temperature
Sunlight hits children more than adults, proportionately: Children have larger heads in proportion to their bodies, and more surface area in proportion to body mass.
Children don't know or understand the symptoms of heat stress, and will play to the point of exhaustion
Thirst is not an accurate measure of how much fluid a person needs, and children rarely know how much fluids they need
To avoid dehydration and other effects of the heat, experts recommend:
Have them drink a glass of water one to two hours before going out, and another glass 10 to 15 minutes before going outside to play
Encourage children to drink regularly, every 20 to 30 minutes; have them drink until they're not thirsty, and then another half glass (for kids under 10 years old) to full glass (for kids 10 and older)
Limit outdoors playtime between 11 a.m. and 3 p.m., when the sun is at its peak
Protect children with SPF 15 or higher sunblock, and reapply often
Dress children in loose-fitting, light-colored clothing made from natural fibers, which "breathe" better than synthetic fibers
Danger in Cars
While heat stress can take its toll on children playing outside, the most dangerous place for children is in a car. More than 30 children died in the summer of 1999 when they were left in vehicles.
NEVER leave a child unattended in a vehicle, even with the windows down. When it's 83 degrees outside, the temperature inside a car with the windows rolled down two inches can rise to 109 degrees in just 15 minutes.
If your child is locked in a car, call 911 immediately!
Also, touch the car seat before you put a child in it, to make sure the surface isn't so hot that it could hurt. A child can be severely burned in just one second. The following chart from the National SAFE KIDS Campaign shows how hot a vehicle's interior can get when the outside temperature is 79 degrees.