I have been looking for a Hitchcock episode now for several years. I saw it many, many years ago.
All the kids in the neighborhood are building strange machines and won't tell any of the adults what it is they are doing or building -- and when pressed they keep saying it's a secret. One of the adults is talking to a relative several thousands of miles away and learns from her friend that the kids there are also playing the same odd game. At the very end of the episode, both the mother and father are terrified of their child and run to hide from him. The child jumps out at them and yells, "peekaboo!"
I was a child myself when I saw this and remember being horrified. It was a great episode and had quite an impact on me. If anyone here knows what this episode is called, it would be greatly appreciated.