To identify series that contain these records, search the Archives Descriptive Database (Archival Descriptions search option) using the keyword guardianship* (including the asterisk), the name of the county or district, and the archival reference code RG 22* (including the asterisk). Alternatively, consult Inventory 22 Courts and related officers records, vol. 1, for instructions on how to identify and access these records.
If on the other hand you are sure it was an adoption, then it does make it harder (but not impossible) to find. The Archives holds no adoption files, only guardianship records. Adoptions occurring before April 8, 1921 were made through an Act of the Ontario Legislature and are extremely rare. For instructions on finding an Act, refer to Research Guide 207: Researching Ontario Bills and Statutes