If your child was born outside Canada after February 14, 1977, and you were a Canadian citizen when the child was born, your child is automatically a Canadian citizen.
On April 17, 2009, the law changed to end the need to retain citizenship except those who turned 28 before the new law came into effect.
So, basically, your child is a Canadian citizen and you can apply for their proof of citizenship (citizenship certificate) any time, provided that -
1) his or her birth certificate issued at the country of birth states you as the father,
or
2) you applied for the child's Certificate of registration of birth abroad (RBA). - which I assume you didn't......
Your marital status before, during and after your child's birth does not matter. It only matters were you or not a Canadian citizen at the time of the child's birth.
http://www.cic.gc.ca/english/information/faq/citizenship/index.asp#proof