It seems Stern Granny missed the 2004 memo from the Supreme Court of Canada that teachers cannot rely upon s. 43 as corporal punishment delivered by them is forbidden.  See Canadian Foundation for Children, Youth and the Law v. Canada (Attorney General), 2004 SCC 4.

Also for parents implements (belts, wooden spoons, rulers, etc.) may not be used when applying corporal punishment.  The use of force must be sober and reasoned, address actual behaviour, and be intended to restrain, control or express symbolic disapproval and cannot be applied to the head.  The force must be transitory and trifling and must not harm or degrade the child. 

They also noted that the child must have the capacity to understand and benefit from the correction, which means that section 43 does not justify force against children under two or those with particular disabilities. Also corporal punishment is not permissible against teens as it can induce aggressive or antisocial behaviour.