Maca and I married when my daughter was 6 and his son was 2. So our kids immediately had 3 parents when we got together. In the meantime, we've had a son and we have a daughter who is biologically the child of myself and GG.
My ex married when Spicy Pea was 8 or so. At that point, she had 4 parents legally.
BUT-from birth, Spicy Pea considered my sister her "other mom" as well.
More recently a close friend has been claimed as an adopted grandmother by my grandson too and with his choice, Spicy has claimed her as a third mom.
I am not at all threatened by metamours being good parental figures for my children. In fact, I'm totally ok with my kids even calling them mom also. The fact is-the kids who were born of me are mine because they love me and I love them. The kids who have claimed me (like my Godson) are mine because they love me and I love them.
Sour Pea is our youngest. She calls Maca daddy and GG "My GG". Her biology is NOT a secret in our home, family or groups of friends. SHe's well aware of the biology. It's just how the names fell together (because she heard the other kids call GG uncle and she knew he wasn't her uncle).
Each child lays a claim to each of the adults in our circle in their own personal way.
Sweet Pea tends to be more possessive about who he calls mom or dad. He never ever refers to anyone else that way. But-he adopts "aunts" and "uncles" easily.
I think that there is MUCH to be said about understanding that love and commitment are the keys to a relationship and regardless if it is child to parent or lover to lover-pet names aren't ever going to mean one person has replaced another.
"Love As Thou Wilt"