Rob Ives has created a fully functional paper safe which is a great DIY project for you and your kids. You can lock and open the secret drawer with your own private combination that is entered with the small numbered dial.
Ives designs paper animations and you can buy a CD with easy to follow instructions, lots of images to show you what to do and the print patterns for the parts. It costs only £2.50 (US$4, €3), which is great value for lots of eco-friendly entertainment and you also get something useful.
After printing onto thin card, use a sharp art knife (adults only for this) to cut out the card patterns. Using glue to keep the parts together, you can part all the separate parts and then assemble them to make the safe. Afterwards, you could customize it with bright colors and keep your valuables hidden from view.