UICenterOnChild.centeredObject tells you what it's centered over. If you aren't using UICenterOnChild, then you need to do math. Knowing UIPanel's worldCorners, use (corners[0] + corners[2]) * 0.5 to calculate the center. Now with the center in hand, run through your grid's children and figure out which one is the closest to that point.
To go to a specific child, use UICenterOnChild.CenterOn, or if you don't use it -- SpringPanel.Begin.