You and your partner have SOs. Your children are comfortable with the SOs in one or both of your absence. Is this A) Relieving (My children are safe when I'm gone, are comfortable with this other person caring for them) or B) Threatening (Oh, God, my children are replacing me with a new mother/father). Assuming the SO in question has made no indication of wanting to replace, but is merely trying to be accommodating to your needs.
Ok, my child had a step parent (K) for two years. I can't see how that is much different from an ongoing poly relationship, if anything, it's "worse" because the child(ren) only see their parents being affectionate with their new partners. I have absolutely never felt that K was trying to replace me. If anything, I did my best to make sure she felt included and wanted. How could she replace me? I'm his mum and we have a warm, loving, close relationship. If, for some reason, I hadn't been a very good mother to him, then maybe I'd worry that she will get in there before I have a chance to salvage things, but that isn't the case so it was never, and wouldn't ever be a threat to me. My son was conceived in a monogamous relationship, but my attitude would be no different in a polyamorous relationship. A metamour could be absolutely no threat to the relationship between a parent and child. Anyone that intends (notice I said intends, because nobody actually could be one) to be a threat isn't suitable to be part of my family.