Responses
I think Mavis doesn't know she had a child I mean she only kissed Zeref and after that she died and Zeref brought her body back to the guild and as far as I remember Mavis went missing when Yury's child had been born and everyone went searching for Mavis but they couldn't found her so I'm assuming Mavis went very far away in order not to hurt anyone so that means Zeref was holding Mavis's body for a very long time so I am guessing that Mavis was experimented by Zeref at that time just like in the game Tekken were Nina gave birth to Steve Fox without her knowing it.
Well, I congratulate you Mavis. Your son is a total Psycho. ╮( ̄▽ ̄)╭