"Where's Max? Where's Max?"
Your dad and I pretend that we don't see you behind the curtain, or in the middle of the floor with a blanket/towel/piece of clothing over your head.
Sometimes we can see your mouth and the smile that widens as the game continues. I'm pretty sure that you know this is a game, and know that we really can see you, but there's no way to be certain.
Then you whip the covers off your head as we exclaim, "Peek-a-boo, I see you!" or "There's you are!" You take a moment to laugh and then pop the cover back over your head.