Kids Alone in Car -What Would You Do?

I really never thought I would witness this, but it happened Friday afternoon - two small children were left alone in a car in a shopping center parking lot. This shopping center has some parking places right next to the sidewalk of the shopping center. My daughters and I were headed toward our car as we walked past a car with a toddler crying. Then we heard a young voice yell "be quiet!" We quickly looked inside and saw two children, about ages one and three, strapped inside the back seat alone. The windows were down. After getting my own kids secured in their seats, I call 911 and told my children why. After just one ring, my seven-year-old daughter said a woman was approaching the car. She got inside the car and I hung up. I did not say anything to the woman, even though she was in earshot. The 911 operator called my cellphone back saying there was a call and a hang up. She asked if everything was alright. I told her yes and why I called. "Is she in the car now?" she asked. I said yes. I knew there was nothing that could be done since she was not caught in the action of leaving her children in the car by a police officer. I thought for a moment of telling the lady she was wrong to leaving her children in the car and she broke the law. I thought of it for the sake of her children in the future. I did not because I feared she might not react too nicely to me or my children - who are my biggest responsibility. Did I do the right thing? Would you have done something different?