I'm actually trying to think about this from a different perspective. I don't have children myself, but I started thinking "If my dad decided to transition and become a woman, what would I start calling him?" Frankly, I haven't a clue. The most promising thing seems like using an alternate language for it. Like, I already call my mother "mum" (not entirely sure how that habit started, I'm not English), and I'm really fond of Japanese, so I'd probably start calling my dad "Okaasan" (mother in Japanese) in that situation.
Another way I'm looking at it is I'm a lesbian, but I would also love to settle down with a woman and raise children together someday. We would both be "Mother," so we'd need to have similar, yet different terms for our children to call us. Again, my personal fascination with Japanese would lead me to choose the alternate language term for myself and be called Okaasan.