Just watched a woman 'supervising' her 4 yr old (estimate) on his balance bike. In the supermarket car park. She was at points around 15 metres away from him.
He had a helmet on. Much good would it do him if she was distracted for a minute or he failed to obey her instructions and carried on behind a car which started to reverse. He was of course far too small to be seen in anybody's mirrors or by glancing over your shoulder.
They picked up the bike and carried it over the road out of the car park. I don't know what they thought the difference was between cars on the road and the cars in the car park. Did they think car parks give cars the ability to sense and hop over poorly supervised children? 
We were meticulous about teaching car park safety with our dc. Held hands, taught them to look out for moving cars. This mother has just taught him that a car park is the ideal place to play. AIBU to think that's a bit of a parenting fail?