I agree that children should be allowed – and even encouraged – to ride on pavements, and they should only start to ride on the roads, especially unaccompanied, when they have sufficient awareness, self-discipline and skill to do so reasonably safely.
However, that doesn’t mean that children should be just left to learn for themselves how to ride safely on the pavement. Riding on the pavement should be where they are taught the beginnings of safe, considerate cycling and roadcraft. That means teaching them how to use the brakes properly, including emergency braking; learning to anticipate and spot hazards and ride accordingly; slowing down and if appropriate stopping when meeting a pedestrian, a dog walker or someone pushing a baby buggy etc. Those skills will then stand them in good stead when they start to ride on the road.
The fact that the child in this accident went over the bars, suggests that they were riding too fast for the conditions, were not adequately aware of what was happening around them, and lacked the skill to emergency brake safely.
I blame the parents.