When I see a child on a "harness or leash-like device" I give a sigh of relief that there's one brat who won't get into mischief or harm by himself.
Today's parents are to blame for unruly children; most youngsters have had little or no home training in good behavior and good manners, which includes staying by Mama's side (at heel) when in public. For the parent it's quicker and easier to harness them.
A leash can control a small child bent on doing his own thing, help make it easier for the parent to conduct business at hand, and lessening worry of damage for the shop owners.
Most dogs graduate from obedience school with better manners than I've seen at times where children are left free to take over.
Maybe someone should open are "Obedience School" for pre-schoolers guaranteed to graduate them without the need of leashes or harness!