Edit: I see Moonflare posted before me, but I think this still might help clarify things.
Yes, the consensus is that she is Akira and Ayane's child. I haven't seen anyone seriously argue otherwise. What isn't certain is her real name. What
Moonflare is arguing for (and I also support this position) is that her given name is Ami, i.e. she has the same name as her cousin/half-sister.
As a preliminary, if you're not already aware, the community refers to her as "Himawari" (Japanese for sunflower) because she tells Akira to name her after his favorite flower, which is later established to be sunflowers. There are also a few references to Himawari in the game, such as HimawariCo.
To be really brief, this is what I think is the strongest evidence for her actual name being Ami:
- Ayane talks about the possibility of giving the name Ami to a future child on three separate occasions.
- During the Untitled Children's Show, Himawari as Ayane says that she picked a name for her baby that she knows Akira would like. As far as we know, Ayane doesn't know about Akira's love of sunflowers.
- At some point (I think in Maya's festival), Himawari tells Akira that her name is "hilarious, a real kick to the shin". "Himawari" isn't a funny name, but being the second person named "Ami Arakawa" would be.
I would say that the last one also makes it definitive that Himawari has a particular name, it's not just up to the player.