SS is now nearly 14, I have been in his life 9 years. When
DH and I decided to get married, we had the discussion about what he should call me. I am not comfortable with a child calling me by my first name I never will be. I believe it is a sign of disrespect and should never be taught (purposefully) to a child that calling any adult by their first name is OK (JMHO). When an adult tries to allow our kids to call them by their first name, it is changed to Miss Susan, Mr. John.
I offered to allow SS to call me anything else he liked as long as it was respectful, and until then he could call me Ma'am (yes ma'am, No Ma'am.) He did that for about a day, and not having developed any creativity of his own he settled on "Mom". We made a deal that I was his "other mother" and that he was to save "Mama" and "Mommy" for
BM only. I also promised that I wouldn't give him a hard time if he slipped and called me "4honor" or Mommy. This deal occurred when SS was 5. I stuck by my deal.
Fast forward 6 years. SS is having difficulties because he has begun voicing his desire to spend quality time with DH instead of BM screwing them out of time together. BM demands "family counseling" for SS and DH and her to work out the issues. During the course of it BM berates DH in front of SS for "making" SS call me Mom. Counselor asks SS what he wants to call me. He says he likes calling me Mom. Tells BM that I may not have carried him in my body for 9 months, but I have carried him in my heart for 6 years. Counselor says SS seems to understand who his parents are and she doesn't see that SS is being forced to call anyone anything. BM quit giving SS grief about what he calls me.
FAst forward another 2 years. We had the discussion with SS when BM was going to get married that what he chose to call BM's fiance was between him and Fiance. (That is a moot point now as BM couldn't hold the relationship together long enough to get to the alter.) BUT SS was grateful that he was not dictated to about what his relationship should or should not be.
Your Skids will call MANY people "mom" in their teen years, their friends' mothers, etc. Allow the kids to call each adult in the mix what the relationship can bear.
Growing up, I had two "daddies". My father and my best friend's dad. But I had about 30 Moms. Though I had 2 step moms, I called only one Mom (and the other one I called an otherwise derrogatory word that starts with B.)
Let your Skids decide what they will call you and do not give them grief for it, cause their mother is obviously doing enough of that.