Daddy is a "familiar" way or casual way to say "Father", "Papa", "Dad". A wife can refer to her husband as "Daddy" once in a while, if he is a Daddy to someone. If he has a child, he became a Daddy when the child was born. Especially if the child is present, or in the room, the child would call him Daddy, and the wife can do that. Just like if the husband and wife are grandparents, she might call him "Grandpa" once in a while, but this would not occur very often. It could be an affectionate "nick-name".

Only in one situation- If her husband is the father of her children, and if she is talking to her children ABOUT her husband.

就像中文的“孩子他爸”）