I can easily answer this question, as I was in that situation a few months ago. We were at a restaurant and a child of perhaps five was running around, speaking loudly, and screaming at times. His mom sat oblivious to her son, and engaged on her phone. This was going on for at least twenty minutes, and our food arrived.
The kid continued his tirade, and management did nothing. At that point I left my seat, walking to the table where he and his mother (still texting) was seated. In a firm but friendly voice, I said looking directly into the boy's eyes, “ Hi, I am here with friends to have dinner, and enjoy ourselves. You are ruining it. Please sit quietly “. He did exactly that. The mom looked at me incredulously, and the restaurant manager came over to say thanks.