This Steel Book Diversion Safe, is a good place to keep small valuable items.
The outside of this diversion safe does look like a book.
If you don’t know it is there, and what it looks like you would never notice it.
The steel safe inside the book cover is lockable, and comes with two keys. If you have things you don’t want your kids being able to get to, you could lock them inside.
Even if they knew you had the safe, they would not be able to get into it without braking it.
Or get one for your kids to hide their stuff. Since it has two keys, you can keep one, in case you want to make sure they are not hiding anything dangerous.