Fact: Even with windows cracked open, a car can heat up to over 110 degrees Fahrenheight, even when it's 60 degrees outside. Little kids develop hyperthermia (high body temperature) way more easily than adults, so your little kid could be DEAD by the time you get back to the car! You want to leave a kid under 6 in the car, here's what you need to do: Leave someone at least 12 years of age or older to take care of them. Lacking such a person, my advice would be to put a children's toy where you're sure to see it. It will serve as a reminder to take your child out. Remember: A kid in a seat could die from heat, and an open window won't change that. Source: US NHTSA - "Unattended Children and Cars"