User blog comment:DanyD/Ezio's Third Child?/@comment-1153722-20140116230429

I actually believe this child must've been born before Ezio's journey to the Vatican in 1499, before he discovered the Vatican Vault. Clay mentioned in the ACR databases that he had access to far less memories of Ezio than Desmond did, and as Lucy mentioned: "In the end, he became obsessed with Italy. I think he knew about the Vault." They hadn't discovered the Vault yet at that point.

If they had, why would Abstergo have tasked Lucy to send Desmond into Ezio's memories, and then sent the tapes of his Animus sessions to Abstergo, if Clay already had access to all those memories?

I always imagined that this first child would've been born during that moment of intimacy with Amelia (that cutscene is extremely awkward with people just walking by a meter away) or one of the courtesans after Marco Barbarigo's assassination.