My daughter has just started to call me mama, she is 18 months old.
First she used "Mama" for drink, I breastfed so I figured that was why. Now she calls it "my Milk" no matter what it is.
At that time she refused to say anything when we asked her who I was. After a while she started calling me "Daddy."
We were up in my Mom-in-law's room petting their cat and she wanted to introduce me so she pointed to the cat and said "Kitty" then pointed to me and said "Daddy", I said "no I'm Mama" and she giggled and pointed to me and said "mama" then herself and said "baby"
After that for about 3 weeks she called me "Daddy" she would say it over an over again to get my attention and I would finally say "Daddy" is at work.
She still slips up at times but now I'm mama.