Interesting project for a child
As @fred_dot_u mentioned above, keychains are a simple, personalized item to make.  In terms of interesting projects, things that I found intriguing when I first encountered 3D printing were objects that could not be made by other means, such as those "geared bearings" (Google: geared bearings 3d print).  Given the popularity of figet spinners a while back, they could be an interesting item.   A captive ball in a box, or three joined rings, can also be set up as a 3D print, and could have the same effect, while being easier for the kids to design. 

EDIT:  Some links to versions of items along the above lines.  Thingiverse tends to slowly break links, though, so these might not last:


Geared Bearing
Captive Ball
Three Rings
Triple Gear
Self-aligning Bushing Keychain