i dont know, if you look at step families or lesbian couples u often times hear the kids call both their moms by different variations of "mother". just because their biological mother is not there doesn't mean they cant call anyone else stepping in "mother", especially one that has been consistantly in their life since birth. if noah wanted that vacant spot solely for summer he first off should not have kept river around but second could have allowed a variation instead, like they can reserve "mommy" for the biological one and "mother" or "mom" for summer. and i wouldnt its too confusing for a kid, they're often times actually much smarter than we give them credit for.
From the start she was a surrogate and those kids got no blood ties to her their DNA is literally summer and Noah's she was just a carrier. From the start noah told her those are his kids with summer and that they will know summer only as their mum and river is just the mother figure that can be there for them and help raise but that's all, they can't call her mum which is understandable but that doesn't diminish their bond at all.
That aside thou I'm irritated with her being weak and wanting noah right now (ch50) but I also understand his is her healing and moving on arc and will wait for her to become a stronger women.
Also hope grey will become ml he'll treat her n her baby so well xo