I think I can speak to both sides of this. I never really wanted children in general. My husband did and I dutifully went through the fertility tests and treatments but it didn’t work out. I was actually somewhat relieved, though I wondered if something was wrong with me because I was relieved. It seemed abnormal not to want children.
Then, when I was 39, I met my child in an unusual way. I will be publishing about that soon. The short version is he appeared, I fell in love with him, and all of a sudden I wanted a specific child. He was my four-year-old friend before he became my five-year-old child.
Sometimes you don’t know you want a child until you meet that child.