I think the artist was chosen for Lachesis because of her age(She's 15 in Genealogy) and because of her original art.
As you can see, her original art is actually not that much different from her Heroes art, especially when it comes to the eyes. It also does not help that she has a FECipher artist that also portrays her as cutesy and young.
So I would say that her designs in both Cipher and Heroes is also likely a choice on IS' part. So it would not surprise me a bit if Lachesis looks "young" in a Genealogy remake. And considering the changes they did to some of the FE2 cast...
The Gaiden designs were just outrights bad for the most part.
But yeah, I guess I can't blame her artist when she's been depicted as young. Though I still don't like the moefication in general.
Though I can't see reasoning for Elincia.
Good God, I remember seeing her in some video and wondering if she got aged up between games but turns out no.
Which is kind of a shame because I really like her artist's art(especially Ninian's) but they just made her a little girl for no reason.