Things like this get out of control because people on the whole are not assertive enough. I find that, "Would you mind keeping your kids under control so I can enjoy my dinner, or should I just speak with the manager?" usually works well. If they get huffy about it, so what? They're in the wrong, not me.
I usualy look at the child and smile at them. In a nice way. I get one of three reactions. One of happy smile back and usually the child will behave if I just look up form time to time and smile or wink at the child. The second reaction is the fear reaction. This comes in two forms. The first being the child where they can react and be quite or cry. Usually their crying though is quieter than the screaming a second before. The other rection is fear in the parents. Then they look at me like what am I doing? At this time, I smile at them. If they let it go, no big deal. If not then I say in my loud voice so the whole place could hear, "I was just looking and smiling at your child, as it seems you are not aware of its' existence or willing to parent the child. So please excuse me, for trying to enjoy my time out, and also helping everyone else at the same time." This usually settles the issue. If not then I just start talking real loud to myself, and or making loud noises, and or screaming myself. Usually the child gets is before the parents that I am imitating them and they stop. Sometimes the parents take offense. I just smile, and ask them if they thought I was talking to them. If so I apologize for them thinking that their behavour matched the description of my rant. The third reaction is being totally ignroed by the child. I then have told them to stop myself. Usually quiet, but if that does not work, I have no problems standing up and embarrsing the parents by saying "NO!" to their child. NOTE: I never touch anyone. I just talk, and or smile or give direction.
I have dealt with a child kicking a seat behind me on a plane, where he would not listen to me or his parent. So, I just dropped my seat as far back as possible and then stared at the parent. The child was picked and taken to the bathroom, and when returned no kicking occurred. Usually just being polite it enough.
So I agree say something, even if it embarasses the parents. This is how they learn.