I recently made the "switch" to TikZ (from xy), and the experience reminded me of the first time I used LaTeX itself.
First, I refused to do it until I had a pressing need to convert (or produce) some specific pictures. Perhaps not surprisingly, the demands of a "live" application motivate a huge variety of seemingly advanced methods and teach you a lot more than doing toy examples. It is also way more motivational.
Second, I relied largely on the tutorials in the TikZ manual. That section is not long: in version 2.10 (which has a new tutorial), it is only 62 pages, and broken into five roughly equal parts each focusing on different methods. They are cumulative, but the manual is well-hyperlinked.
Third, I spent several days, where the first day was basically getting just one picture to work. I went back to the manual every few minutes and pushed myself to make the code "right" even if I had found working but ugly code earlier. As I figured things out I applied them retroactively to previous pictures and, in the process, developed a good sense for what works when/where. I also built up a "style file" of TikZ styles (many defined by myself) and libraries which I always used.
Fourth, I used this site a lot! If you want to know the sorts of things I wanted to do but couldn't find in the manual, have a look at my questions here (go chronologically or, roughly, in decreasing order of upvotes as they became ever more esoteric). By now there is a good chance that a given basic question has already been answered, which will save your students even more time. If not, well, they should ask here, which is a valuable skill in itself.
There's no royal road and all that, but it sounds like your students are in the perfect position in their TeXing to take this particular leap. If you're willing to devote some of your own time, I can say (from my experience as a grad student, not necessarily in learning TikZ) that it would be a huge help to critique their code and their results for a little while. The right way to do things is often the easiest in the long run anyway.