I know it's hot, I know you might not want to get out, but you can't protect your child from the inside of your car.
Someone could snatch your child and run. You won't be able to catch up. Your child could wander off and get hurt. Hell, they could fall and bust their head open and it's gonna take you at least a minute longer to get to them.
It's the park. Get out and enjoy some time with your children while they're still little.