I have two DC, and DP has two DC.
From the point of view of his DC. I don't refer to myself as their sm, because I'm not there yet. I don't think it's anything to do with us not being married, or their DM. I think it should come naturally as the relationship builds. They are obviously family, but I think the relationship needs to develop more. I do do things with them, but we tend to do things as a family. I'll entertain one while DP bathes or dresses the other, I'll run the bath, organise towels, help them pick a soap, or bath bomb etc. But he does the actual bathing. Same with getting dressed, I buy the clothes, help them pick out their outfits, leave them on the beds, but he helps them get dressed. I know he would like me to be more involved, and so would they, but I don't feel that the relationship is at that point yet.
That's ignoring that communication with their DM is tense, and argumentative on her part, which does make things more difficult all around. She doesn't yet have issues with me doing things specifically, more that she wants us to use her very strict routine, and we have our own. We have similar mealtimes and bedtimes, but we don't set times for specific activities, such as 10am fingerprinting.
From my pov as a parent. My DD is 12. There's things that her DF can't do that her sm can. DD wouldn't be comfortable asking her DF about periods or stuff like that. She has a code word for DP, so if I'm not around when her period starts she can just say the code word, and he'll know what's happening and how to deal with it, without embarrassing either of them. But she didn't want to do the same thing with her DF, she'd prefer that sm helps her if and when the time comes. Why would I want my DD to feel embarrassed, and uncomfortable, just because someone who is not biologically related is taking care of her?