I live in canada .I'm not sure how the rules are here.
Is using physical force on children an exception?
Yes. Section 43 of the Criminal Code of Canada allows the use of some physical force if the purpose is for disciplining a child under the age of 18. Only parents or people who are in the place of a parent (for example, a step-parent) can be excused if they use reasonable force on a child for discipline.
The Supreme Court of Canada studied this law in 2004; the citation for the case is: Canadian Foundation for Children, Youth and the Law v. Canada, [2004] 1 S.C.R. 76. This handout explains the rules that the Supreme Court of Canada made on when and what is legal when disciplining a child; this is the current law in Canada.
http://jfcy.org/en/rights/corporal-punishment-aka-spanking/