Yes, I was leashed once. When I was three, we wre on a campsite, and the playground was just around the corner from where we set up our tent, just out of sight. So my parents put a rope around my waste, with according to them, a lot of slack and then I, being the little bastard I was and still am, stood right at the very end and cried on the top of my lungs. Then, they removed the rope and took me to the playground.
Sort of a childhood trauma, so no, leashing a child is not accptable under any circumstances.