I think the title should have being phrased more accurately and sensitively. Should have being "A child being restrainted by a body harness".
"A chlid on a leash" tend to conjure up a picture that the child have a "doggy neck collar" on him. So the first picture that a person have, would be the child is tied up like a pet dog.
Maybe the parents want the child to have the freedom of roaming around when the kid still do not know the dangers that is around. In SG, the dangers would be vehicular traffic, or the kid wondering off and get lost.
As opposed to having to carry the child or restraining him in a kiddy stroller (the pram chair with wheels). I have seen some kids who are very large already still sitting in a stroller, looks equally strange. Conjures up a picture that the kid is spoilt rotten.
Maybe the kid is just very naughty, and refuse to be taken around in hands. Sometimes being a parent is very very taxing.
Thus this as a commonsight in european countries, I don't see anything wrong, they are more liberal anyway. It's meaning can be taken both ways, how it's being used is the most important question.