http://www.cic.gc.ca/english/information/applications/guides/5269ETOC.asp
Minor children who are less than 17 years of age and come to Canada to study without a parent or legal guardian must be cared for by a responsible adult in Canada. This person is known as a custodian. A notarized custodianship declaration, such as the form Custodianship Declaration – Custodian for Minors Studying in Canada (IMM 5646) (PDF, 1 MB), must be completed by the parents or legal guardians in the country of origin and by the custodian in Canada.
From your previous posts, you are coming as a student and your children are accompanying you, so IMM5646 is not required.