You do not need license if you have less than 5 children under age 10 other than your own.

See http://www.edu.gov.on.ca/eng/parents...package-en.pdf

"A person may provide informal child care to five children or less under the age of 10 years who are not of common parentage (children who have different parents), in addition to his/her own children, without a licence. This number may not be exceeded, regardless of the number of adults present or on site."