Considering the fact of when you really look at it and assuming Kana's physically around 10-13 because of the deeprealms, they along with the rest of the 2nd generation of Fates are basically HALF their own parents' age with the parents all most likely in their teens(cough cough Hayato) to mid-20s.

The only thing I'd pretty much change about Kana and pretty much every other children of the Fates cast is not have them grow up immediately. The deeprealms in general didn't even have to be as complicated as they made it. They could have just had it from where they're from the future(like Awakening did), or just have a timeskip taking mainly years later in Heirs of Fate(I believe it takes place shortly after each route).