Of course they can be down on a knee in a bunt attempt, but as Geoff has pointed out, in softball they may not be in contact with the plate at the point of bat contact. This is not true in baseball where they can be in contact with the plate with their foot or their knee as long as that foot is still also in contact with the batter’s box (or the line of the batter’s box which of course is considered to be part of the batter’s box), and if they make contact with the ball, they are NOT out.