If you can get hold of one, I think a good ol' four track cassette tape recorder is still a great way to go (e.g. Tascam Porta-02). The main disadvantages are that cassette tapes are harder to come by these days, the sound will be a bit crackly and in order to perform the mixdown you'll still need to plug the 4-track output into something else.
But for these minor setbacks you have a physical unit which is simplicity itself to get going with and custom made for the job of introductory recording/mixing. The two most fundamental skills are covered:
- Recording tracks sychronously while listening to previously recorded tracks.
- Mixing down the four tracks to a stereo sound, using volume and balance controls in real-time with some LEDs to guide you to the right level for L and R channels.
Everything to do with the recording process is limited to the physical hardware so they don't have to worry about peripheral issues such as sound drivers or VSTi configuration. The skills will easily translate to more sophisticated DAW when they're ready (it did for me).