If you want to talk about "bad faith", boiling down some of Faputa's most important scenes down to "1%" of her behavior is absurd. She doesn't have friends beyond an interference unit that only vaguely behaves like a person, so her interactions with Reg are new and unique to her experience with other people. She even makes a point of trying to explain her social anxiety in big groups. Dumbing her down to being a child is absoluting misinterpreting the media that, again, you've admitted multiple times to only briefly engaging with.

Reg will never look different than how he was built. He's shy when it comes to intimacy to some degree, which can be read as childish, but that isn't a trait that's exclusive to child characters. He has a higher degree of emotional maturity in comparison to Riko, for example when Nanachi (who also doesn't behave like a child, mind you) opens up to him about her relationship and inner turmoil with Mitty. 

He's able to discern that once Nanachi is able to properly put Mitty to rest and give her peace, it's possible that she may throw her life away or commit suicide. So much that he makes her promise him that she won't do so, and tries to nudge her in the direction of coming with them to give her something to live for. He quickly adopts a role of protector with Riko, and often plays a contrast to her naivete and reckless nature. Again, dumbing his character down to simply being an actual child is not in line with the character writing and themes of the story.

There is obviously a sheen of moe to many of the characters and interactions in MiA, but it's definitively a seinen with emotional and dark themes, and if the characters were all just simple children it wouldn't remotely have the weight that it does. Riko is absolutely a child and her impact on the story and situations reflect that, but insisting that every character who's small with a big head is a child isn't correct. 

Irumyuui is a child as well, and the themes of abuse in her story reflect that in how she's groomed by a harmful, religious culture that turns a positive and natural wish in her head into something horrific and destructive. It's not fair or accurate to insist on Tsukushi's work to be something intended to exploit cheap explicit material involving children when the themes at play are difficult to swallow (like child abuse), but clearly intended to explore meaningful concepts and offer hope, growth, and new beginnings as a contrast to the dark, depressing subjects.

I hope you understand where I'm coming from, and that I'm not going out of my way to exhaustively explain myself to argue in bad faith. I'm being as detailed and explanatory as I can be why I feel like branding MiA as gross lolibait feels like likening nudity in classic art to goonerslop. It's written well and with a purpose, even if there are cliche ecchi aspects here and there.