Following each narrative, children were asked to judge whether various bodily and mental processes continue to function after death, and to justify their judgment. Although children often claimed that functioning ceases at death and offered appropriate biological justifications for that judgment, they also claimed that functioning continues after death and offered appropriate religious justifications. The tendency to invoke an afterlife was more frequent among older children than younger children, more frequent in the context of the religious narrative as opposed to the secular narrative and more frequent with respect to mental processes than bodily processes. Particularly among older children, two distinct conceptions of death appear to co-exist: a biological conception in which death implies the cessation of living processes and a metaphysical conception in which death marks the beginning of the afterlife.He probably knew very well that he wanted her and the baby to go away, perhaps permanently. But children's understanding of the permanency of death, and the future, is very different from ours. They can "understand" things, but they understand it differently than we do. The brain develops a great deal between 14 and 18.Given all that and the severity of the crime, there's no reason to pretend he didn't know what he was doing.
The Understanding of Death