Since this is a Harry Potter fan club and you want to teach people about rationality, shouldn't you, IDK, tell them about HPMOR or something?
HPMoR page has a link to Spanish translation of the first five chapters... but the link doesn't work. Perhaps google could help?
Then if you are willing to spend some money, you could print a few pages (I guess including Minerva's cat transformation, but not including rape discussion with Draco), bind them together, make a few copies of that, and give it to the fanclub members. It is much easier to read from paper than from the screen. And when they read the start, they will be more likely to read the rest online in English.
Another idea: How about making a ...
At my local Harry Potter fanclub (Bogotá, Colombia) some members teach "classes" on subjects they're passionate about. We've had informal courses on history, creative writing, English, etc. But recently some other classes have appeared that have made me worry seriously: astrology, divination, ancient runes, and all other sorts of nonsense. They're not taught as folklorical pieces of the past, but as serious practices that are supposed to actually work. I think this is particularly dangerous for the small kids that comprise the majority of the fanclub and still need help learning that magic doesn't exist.
So I proposed the fanclub chief that I could teach a Muggle Studies class: logic, critical thinking, philosophy of science, etc. In two weeks we'll have our first class, and I intend to begin talking about the most common biases. I already downloaded this website's PDF guide to holding a Less Wrong meeting. Aside from that, what can you suggest for a successful Muggles Studies course?