Too much waiting. They need to be born, learn to talk, understand the differences, make a choice... The MC will call Martha's first daughter Sharon until she chooses. If later "she" wants to be called Shawn, there will be no problem. Calling "her" Kid#5 until then is not an option, "she" is a kid, not a Mambo.
It's easier, at least for me, to think (and write the epilogue) about the MC's children as:
Kalyani: X sons, X daughters, X "children"
Hailey: X "children"
Nia: X daughters
At the end of the day, the epilogue will take place once they have chosen, so the MC will refer to them as their chosen gender.