I've called the police a few times when I've seen kids bouncing around in cars driving down the road. That kind of thing drives me nuts. Even if nothing comes of the calls, it makes me feel better, at least I've tried. You did the right thing to call it in.
Once I got behind this car with what appeared to be a 3 year old with her head and shoulders outside the back window, going through Waldorf at about 45 mph. Couldn't believe what I was seeing. There were 2 other little kids bouncing around in the back seat too. So I got up behind this jerk and wrote his tag number down to call him in, and I guess he figured out what I was doing because he gave me the finger. I called the police, followed them, and watched them park at a Karate place. I was still on the phone with the police and told them where to find these people. The driver saw me and yelled at me to mind my own f***ing business. So I yelled back, "If you cared about your kids you'd have them in seat belts." The woman with him then said, in her mournful, I-can't-do-anything-about-it-way, "You're right, I keep trying to tell him but he won't listen...."
So I waited to see what would happen (parked across the lot), and yep, the police did show up, within 10 minutes or so. It was gratifying to watch the driver being questioned outside by the police officer. I don't know what came of it (I think the officer has to actually see it to ticket it), but I hope it made him think twice about not buckling their kids up again. He could have hit a pothole and that little girl would have just flown out the window, it gave me the willies just thinking about it.