Unless the kidnapping happened by random chance, it must mean the victim could have done something to alter the risk of becoming a victim. To what extent is knowing this but not acting on it a form of personal negligence?
There's no law to say at what age a child can be left unattended (at home), but it is an offence to leave a child unattended if it puts them at risk. That depends on how old the child is I think - at an age where a child has a sense of 'stranger danger', safety and knows what the rules are, where they can go, etc etc etc, I'd say it is reasonable and not negligent to leave that child unattended. For very young children that is obviously not the case.