I found a company called Window Monsters. They make windows. But Window Monsters… Hm… That gives me an idea…
I could make the life-sized hideous monsters and call them Window Monsters™. They’d ‘live’ below a window, outside. After nightfall, whenever the light switch in the bedroom is off, they will rize up via a mechanical device and ‘stare’ into the window. When the room light is on, or when it’s daylight, they remain hidden. Perhaps they could have some subtle lighting or glowing eyes or something. Maybe they could even make noises.
This would be perfect for the kiddies’ bedroom! And it would go great with a Perching Demon Life-Sized Crib Figure™.
My grandparents’ house had a Door Monster in the living room. It was a pattern in the wood grain on the door that led from the living room to the porch. To an imaginative child like yours truly, it looked exactly like a monster opening its mouth wide to eat somebody.
The OP reminds me a great Far Side cartoon that shows a kid upstairs in bed, a parent downstairs in an easy chair, and a string running from the parent’s hand, out the window, and up to a giant balloon hovering just below the kid’s windowsill.
The balloon has a big scary face on it & the words are “Billy, now you be quiet & go to sleep, or I’ll have to summon the Giant Head of Doom again”. The kid is not looking happy.
One step farther(further?): LCD window clings. You know those electronic LCD window shades? Switch 'em on they turn black, switch 'em off they’re transparent? You make a window cling that behaves the same way, except instead of just turning black it turns into the desired image. And get this: You make it solar powered so when the sun goes down it changes into the monster–which evaporates immediately when a parent/kid flips on the light…just like a real live monster.
I don’t want much of the profit, Johnny. (Hey! Johnny Profit!) Just the MGB company car.