After some web surfing, I discovered: they exist. They're called puzzle boxes.
And sneaky grandma that I am, I got one style of puzzle box for my magic and a completely different style as gifts for my grandchildren. This way I get to introduce them to the idea of puzzle boxes. They get to have their own puzzle boxes (with a prism hidden in each one). And my magic will be safe. A child trying to get a quick sneaky peek into a box will know what s/he's up against when the box does not instantly open.
But, I could be wrong again. A quick peek on eBay shows that many sellers are getting rid of gently used and new-in-box puzzle boxes. At least nobody can undo the magic of prisms. They always make rainbows in sunshine.