I did it once. I had three LOs in carseats. I parked, turned off the motor, rolled down the window an inch, locked the doors and walked a few steps to an ATM. My children could see me and I could see them. A police car drove up and gave me a ticket for child endangerment. Great. Story of my life.
Statistically speaking, I'd lay money that your children would have been at greater or at least equal risk getting in and out of the car, walking to and from the ATM, and waiting at the ATM while you withdrew money. I'd be curious what the outcome of the ticket was since I do this all the time.