CAM recommendation from a different angle

[Stepper Monkey]

I have kind of an odd question. Forget cost, forget capability, forget features. What is the easiest, most intuitive program to teach kids and old people who are neither machinists, designers, or even particularly computer savvy?

I need a CAD/CAM design program, or pair of CAD and CAM programs that work well together, to bundle with a small (like Nema 11 small), dedicated, single output machine.
Technically it is a form of CNC mill, but its users don't know that.
I am limiting it's functionality, fixturing, and envelope to do one thing only - cut user-generated reliefs into the top surfaces of identical pre-sized plastic medallions, and do it as simply as possible.

In short, it is an appliance, not a tool. Call it a 3D printer for simplicity, a high-tech toy.

As such, what is the best way to get user-generated 3D reliefs created and then transferred to g-code, when the users are complete idiots?

The g-code shouldn't be a big deal, as there will only be one type of supplied bit on a machine whose hard limits are the one shape of plastic disc we supply.
Generating the relief, however, could be a pain. I need to be able to strip down a program to its simplest and least complex form. None of the customers are gonna want to learn any more than they have to.

Is there a simple relief generating program? Is there a more competent program like Rhino, etc. that can be configured only to show a small fraction of it's true capability - i.e. a "skin" with almost all features removed?