The one thing I'm worried about is that the defense will actually use the murder of that poor child as a defense.... ie-- What kind of person murders a 5 year old child?... A mentally unstable person, that's who.
A very real possibility (with many precedents in other money-motivated crimes as this one appears to be from information available so far) is that the child's death was accidental. That is, the perpetrator intended to silence the child, tie him up, carry him out to the truck (or something similar) and accidentally smothered him, dropped him or otherwise miscalculated.
There are plenty of precedents for this kind of thing (two that leap immediately to mind are the kidnapping of Cecilia Zhang in Toronto a few years' back, and eons ago, the kidnapping of the Lindbergh baby. Cecilia's abductor put a towel over her mouth and nose to prevent her making noise that would wake her parents; she was asphyxiated before the assailant realized what had happened. Of course it was still a homicide; eventually the person pleaded guilty to second-degree murder. The Lindbergh kidnapper (probably someone other than the man executed for the crime) only wanted to take the baby for ransom, but apparently accidentally dropped him from the second-story window when the ladder he was using broke as he was climbing down with the baby. The child suffered a severe skull fracture.
From what we know of this accused, he had little or no experience with young children and would not have been intuitively aware of their fragility, not only re respiration but their larger head:body weight ratio makes neck and skull fractures much more likely than in adults.
Not saying this happened, but it is a possibility -- there may have been no intent to kill the child (his value as a "witness" would have been negligible -- a child that age cannot give sworn testimony) but caught off guard, possibly panicking, and with little knowledge of how to handle a young child, the assailant could easily miscalculated the force etc. needed to subdue or silence the youngster.
Canadian law is not the same as U.S. where a death in pursuit of a felony is automatically a first-degree murder (reasoning that since the offender intended the felony, s/he also intended/premeditated whatever additional mishaps occurred), but I think -- would have to look it up -- that the criminal circumstance automatically makes such an accident a murder, not a manslaughter, event under the Criminal Code.
Being mentally unstable does not get someone off the hook, however. A history of schizophrenia (where an individual cannot discriminate between reality and hallucinations) is different, but does not seem to apply here; nor is there a suggestion of neurological disability such as autism, fetal alcohol syndrome etc. that can impair a person's everyday functioning. This individual has been able to hold down responsible jobs, engage in entrepreneurial businesses, etc. -- a case for "not criminally responsible" would be a hard sell, especially if there is evidence of planning and preparation.