R.K.
We didn't have cell phones when my kids were little. If I took them to a crowded place I would buy them a balloon and tie it to their wrist. If they got ahead of me I could find them easily in the crowd. I just looked for the balloon.
Seriously though I would never allow my child to wander too far away at a carnival or other public event. Keep them close and keep an eye on them. My youngest, I had a harness and leash for, people made remarks about a kid on a leash but he pulled against my grip on his arm or hand and I was leaving marks on his skin trying to hold on to him. So the leash it was and I knew he was safe.