Could it be that Chao mentioned Negi's past (how Negi met his father 6 years ago), because she wants to change/preserve it? Could it be that's the reason she leaked information during the mahora tournament? To warm the world up for the exposure of magic (by Negi)?
Maybe (it's just speculation, but that turned out to be quite accurate lately) Negi did not meet his father 6 years ago. We know that Negi can have (a somewhat) adult body thanks to Kamo's pills. Chao might just want to send Negi back to the event 6 years ago and have him act like he was his father. It's a very important event for Negi (it's the one that made him decide to become a Magister Magi) and if it did not accur, then maybe the world from a 100 years from now would be in trouble. because Negi would not strife to be a Magister Magi. Maybe the world will know of magic thanks to Negi. I mean, like half of his class already knows about him and magic, so why not?
It could be that Negi will pass such information down his bloodline to make sure Chao would go back in time. (And maybe the colonies on Mars will be possible thanks to hybrid science-magic, like Chachamaru)