You know, this could be the seed for a really interesting story. I mean, the show had that thing where their bodies are really puppets and they used it as an excuse for a freak-out, but there are a lot of other ways that you could go with it. For example, they could come to accept it, either because they found out and they're OK with that kind of thing or because they lived long enough that they had to accept that something was up, and treat it like any other form of transhumanism... In a world where no one else is a transhuman.
I know this isn't what's happening in this story, but it would be really cool if Mami had a childhood friend that she grew up with and eventually started dating. Except, the friend would keep growing without her and eventually you'd have something like the scene here, where they both love each other and are perfectly mature, but one of them still has the body of a child. It would be interesting to see how that would effect their social lives, both in terms of their normal friends and the way strangers would judge them, and how they would deal with the yearly reminder that eventually one of them will grow old and frail and eventually die and the other simply won't.
Actually, that could be set up as a very interesting prequel to the actual events of the series.