The most common advice for them would be to get a prime lens. Especially in this simulation that offers both distance and focal length setting (which are, for this scene and 18-55mm focal length, mostly interchangeable), I'd like to have it. It'd also allow you to go (cheaply) to f/1.8 or f/1.4, which would be much better to illustrate the effect that aperture has on the depth of field. With a common f/3.5:5.6 kit lens and APS-C sensor, you won't get a shallow DOF and nice out-of-focus background easily, and that could be frustrating to a beginner. Especially when you learn to do it in this simulation.