I wouldn't do it. What if someone broke into the car while you were inside? An unlikely but not impossible scenario. I know of a family whose 2 year old was asleep in the car, so she was left in her car asleep in the drive, and the car had a freak electrical fault and burst into flames, and the mum couldn't get her daughter out. Again, totally unlikely but has stopped me from leaving my dd alone in the car.
Is it that much hassle to take her into McDonalds with you?