Right. Now given a situation involving an adult pedestrian behaving unpredictably and a small child learning to ride a bike, or a grandmother with not the best of reflexes riding leisurely, would you agree the person bringing the greater risk is the pedestrian? Just trying to establish that pedestrians also have responsibilities and a collision by itself does not prove the cyclist was at fault.
Not sure whether you are after the legal or the moral position. The legal position as I understand it (IANAL), is that the party that claims to have been injured due to the other party's actions would need to prove negligence on the part of that other party.
As far as the scenarios are concerned, there is not really enough information really to make a firm judgement.
However, in the case of the old lady I would say that (again, IMHO), in the absence of any other factors, she is at fault as she should not be riding in this place if she doesn't have the reflexes to cope with it and is therefore being irresponsible. It is analogous to some doddery, half-blind old guy who hits someone in a car; it's his fault because he shouldn't be on the road. (And there are many examples of the latter.)
In the case of the child, I think the parents are being irresponsible having the child ride in a place that they are not yet ready for.
The person bringing the biggest risk to these situations is the person that brings the bicycle there.