In Riven, after you have advanced to a decent point in the game, go to the chamber where you first appeared in the game. In that area, go to the edge of the cliff facing the holograph room and the first transport. Look down the cliff and you will see rocks and the water. In the upper-left side of the screen, in the cliff, is a photograph of two "MYSTerious" children. Those are Robyn Miller's children.