First, I wanna show you this cover. It's kinda symbolic. This was the cove page after the Behelit monster ate the baby.
Found it!! This is the page. As what you can see the fetus has now turned into a full grown baby.
Remember when Griffith was turning into a demon god, he was also inside a Behelit. And the tower arc was kinda an imitation of the eclipse feast. It bears a twisted similarity to Christianity. The son of the demon god, born from a mortal woman, is also the demon god himself.
I agree with you regarding the other kid. I do think that he must be Caska's and Guts' kid, just born later.