Beyond being weird and strange, this is just plain wrong! If you and your wife want to partake in this creepy then fine, but leave your other living children out of it. What if this screws up the other three children? They just took a traumatic experience for them and turned into a possibly very traumatic and life shaping experience for their other 3 children. I truly believe that the experiences we have when we are very young shape us for the rest of our lives, children's minds are sponges at this age, it's amazing how much influence these early life experiences we have effect us for the rest of our lives.