Lore

Wait? Birren had three children? Well, he saved three, but I don't recall all of them being his. Also, wasn't Dian the leader of the settlers? Not some slave's (Birren) kid?

I realized that the symptoms those three children experienced was a curse on the settlers, a curse from god, a curse of death. - Complete the Passage

But this man had been able to cure his children. The slates specifically state that he saved his children, no one else.

TH, while reading the slates, mentions that someone else wrote some parts, these are most likely by Dian. Meaning someone else was there way before her and TH talks to Dian mentioning "our mother/father".

I'll concede on this point.
Any hints as to who that last kid is? TH, Dian, and someone else...

Nothing about the last kid. It's possible both TH and Dian don't know if the last kid is even alive. Considering Dian didn't even knew TH was alive.