You will have to claim common-law. When you are living together and have a child, the date of becoming common law is the date that the three of you started living together...this could be the date your child was born, or the date you and your partner started cohabitating, if one of you already had a child. Please review the link below.
http://www.cra-arc.gc.ca/tx/ndvdls/tpcs/ncm-tx/rtrn/cmpltng/prsnl-nf/mrtl-eng.html