Your child is vulnerable. They do not have matured safety skills. They do not always understand when they are in danger. When you leave them unsupervised in public, you are putting them at way more of a risk than just heat. It isn't okay in winter, spring, summer, or fall.
Don't make excuses of why you can't take them in somewhere. You need to make accommodations or learn how to handle your children. Leaving them alone in a car should never be an option. Trust me, they are worth the hassle.