 A 10-year-old boy died last week in Phoenix after a hike on a 113-degree day. A four-month-old girl died Friday after being taken on a boat on Lake Havasu in 121-degree weather. And a two-year-old girl died Tuesday after being left in a car near Tucson when the high was 111. Dr. John McGreevey with Phoenix Children's Hospital says kids' bodies can be very vulnerable to extreme heat.