At the beginning of your upkeep, sacrifice Child of Gaea unless you pay.
: Regenerate Child of Gaea.
10/4/2004 You choose whether to pay or not on resolution. If not, then you sacrifice Child of Gaea. You can choose to not pay even if you no longer control Child of Gaea on resolution.
10/1/2009 If you do choose not to pay, but no longer control Child of Gaea, you cannot sacrifice. It will remain on the battlefield under its current controller’s control.