Most of my DCK are little ones that barely walk so of course they are supervised at all times. I have two 3yos (one who is my own) who I trust 100% to play in another room. They do puzzles, read stories, play barbies and do quiet crafts while the little ones are sleeping. I completely agree that it depends on the child, not just the age. These two play amazingly well together, both understand they need to be quiet during nap time and relish being "big" enough to be trusted.