Our local library makerspace holds summer camp for a limited number of lucky attendees, ages from 12-15 and the curriculum has a segment which appears to match your objective.
In the case of the Launch Pad Camp, the campers will be using OnShape to create a keychain/nametag with text of their choosing. It will be an unremarkable 3mm thick x 35-40 mm wide x 70-80 mm long "plank" with a depressed center (by 1.5mm) and raised text. The mentors of the camp expect to be able to teach these campers how to use the software to perform tasks by rote. I assisted the mentor in the process as she had no idea how to use OnShape. Neither did I, but that never slows me down.
Another aspect of her camp is that it will (or may) include using Scratch
I suggested to her that she could use BlockSCAD to create the 3D objects, as the interface is nearly identical, and in my opinion, very easy to use.
Not surprisingly, BlockSCAD has a series of tutorials available on the YouTube.
The editor for BlockSCAD is browser based removing the requirement to install on every computer.
A member of our hackspace wants to get their 8- and 11- year old kids1 excited about 3D-printing and CNC cutting and makering in general. We have a weekly open training where people can design and print/CNC/laser an item of their choice like a dogboned box or a two-piece sword and hilt, or a name tag, or whatever. I need a few ideas prepared so we don't spend half our lesson on Thingiverse or in Fusion (more like a quarter)! I'm not a parent and my youngest friend is probably in their twenties.
What we have:
- a full bed 1200x1800 mm CNC (preferred, because it's loud and fast)
- Prusa mk3
- cheapo 80W 500x300 mm laser
- Fusion 360 based workflow, easy :)
- Arduinos and stuff
Ideas we've had: a minecraft creeper, done as a simple-ish box.
1: the (girls) are not interested in my normal kids' goto, which is: swords, shields. They are interested in: minecraft, dragons, horses. _o_/
edit: this is NOT an opinion-gathering post, though there may be more than one "correct" answer. We need specific applications of 3d printing for a young audience. This collection of answers will be useful to evangelize making to a whole new generation!