Someone suggested Maxwell render above. I have never used it so you may try their free version. What I was able to start rendering at least fairly acceptable results was Twilight Render (which actually uses Kerkythea's rendering engine built into SU and running inside SU). Its interface is simple and intuitive enough even for a beginner at rendering.

It has a free trial version with limitations: no export to Kerkythea, size limited to 800 pixels and some watermarks in the corners. But they also offer academic licenses for both educators and students: http://www.twilightrender.com/buy-top/86 (full price would be $99).

The fun part of Twilight is that it can render special SketchUp animations like SketchyPhysics.

In any case, you can start with the trial version here and see if it fits your needs. Keep in mind however that any of the renderers will always benefit from more powerful computers. Twilight only uses CPU but of course, the faster your CPU is or the more cores/threads it can render with, the faster your images will be.

Another option could be LightUp which also has some trial version although I am not sure what the limitations exactly are: http://www.light-up.co.uk/index.php?t=download They also have academic license: http://www.light-up.co.uk/index.php?t=student

The cool thing in LughtUp is that once you render your scene in SketchUp, you can interactively walk about that scene and even export it into a standalone player (or publish the walkabout scene on the web). I am sure kids would love this (and to how off to their parents/friends, too).

So there are many possibilities to get them acquainted with. Christmas is coming - maybe a license of some of these lesser expensive apps is affordable for some families. 😄

@adorno said:

The recommended system requirements fo Lumion are very high though:

OS: Windows 7 64-bit
System memory: At least 6GB
Graphics card: NVidia 460GTX / ATI 5850 or faster, with at least 1GB memory

True that since it's a GPU based engine,it needs a stronger machine. I use it on a 3.5 year old, 32 bit XP though (i.e. max memory usage is less than 4Gb) with an 1 Gb nvidia 9600 GT which was not a top notch card even when I bought my PC.

(I know, time to upgrade...)