You didn't tell how old the small kids are. This probably matters. 7 year olds are different from 11 year olds are different from 15 year olds. Whatever you pick as the topic, you should probably come up with a treatment that simpler than what you'd think necessary. And then make it even more simple.
This is something I've struggled with (am still struggling with, really) in my time teaching kids as a volunteer. I already knew the basics of child psychology, remembered more or less what concepts I could expect kids to handle at various stages of development and tried to develop lessons which accounted for their abilities.... but in the beginning, I still way overshot for the younger kids, because I failed to keep in mind that honestly, normal little kids are really dumb. If you treat them like teenagers minus some major reasoning faculties, you're still going to seriously overestimate the caliber of thinking they're likely to be capable of.
Which is why, even allowing for suspense of disbelief, Ender's Game is so ridiculous.
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?