First, I wanna show you this cover. It's kinda symbolic. This was the cove page after the Behelit monster ate the baby.

http://www.mangareader.net/96-1140-2...hapter-20.html

Found it!! This is the page. As what you can see the fetus has now turned into a full grown baby.

http://www.mangareader.net/96-1141-1...hapter-21.html

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.