Step-parent, aunt, Jennie, Pop, Ma’am, Mr. Smith . . .
Basic rule of sanity: kids need a label for the adults in their lives. A box to put the adult in so they can know what their relationship with the adult is. Any time your kids ask what to call your SO, what is really being asked is ‘What is this person to me?’
So before starting to think about what your kids should call your SOs, take a minute to think about this. Just what is the relationship between your SOs and your kids? Are you raising you children from birth in a group marriage and all the spice are parents? Are you going to be introducing your teenage son to your girlfriend for the first time?
Group marriages who are raising children together tend to take one of two approaches to what I call ‘parent names’. Either the non-biological parents choose terms from foreign languages/other cultures that appeal to them (Ima is Hebrew for mom, Padre or Papa from Spanish, etc) or else the non-biological parents are Aunt or Uncle and only the bio parents are called anything related to ‘mother’ or ‘father’. Raising children in these situations, it isn’t something you ever need to discuss with them, the same as you never sit down with your toddler and say “I am your mother and you can call me ‘mom’.” You just walk into the room saying “Hi baby, mommy’s here!” and eventually baby learns that ‘mommy’ means you.
If you are introducing an older child to an SO for the first time, you probably want your kid and your SO to get along, but unless the SO is moving in with you or something, they don’t need to be particularly close. So don’t make it complicated. As long as your SO agrees, you child(ren) can call them by their first name. No reason to make a big deal out of it.
Things can get a bit complicated with you are ‘marrying’ an SO (whether legally or privately) or you are moving in together. Either when someone is moving into your home to make a triad/quad/etc, or you’ve been living alone and will now have a primary partner moving in. In these case, you might want to do some research into the advice given for helping kids adjust to having a new step-parent, almost all of it will apply. The short version is: let you kids know you want them and your SO to have a good relationship, but that relationship is up to them. They can start out calling your SO by their first name, and if later they decide they would like to call your SO aunt, uncle, Pop or something else, that is up to them. The message you want to give your child here is that they get to choose the label – the relationship they’ll have with your SO is up to them, and they won’t be forced into a relationship they aren’t comfortable with.