In German cities we have Repair Cafes and Makerspaces (Hackerspaces, Fablabs). Many of them are known to happily help out with 3D-printing. Maybe something similar exists in you area?

We really like this treasure chest https://www.printables.com/model/206656-treasure-chest-puzzle-box-update. Mechanism works very well and children were exited when solving the puzzle (as present box). There is even a video with its maker showing the variations and explains the unlock mechanism.