Sir Cumference and the dragon of pi : a math adventure

In my opinion, this entertaining book does a fairly good job of introducing the pi ratio to younger kids. However, my son's 2nd grade teacher read it to her class and they didn't quite get it. For example, one girl thought the ratio was 7 (instead of 3 1/7), because in the final example the diameter is 7. We then did a hands-on activity from the Exploratorium web site, which is a good way to reinforce the concept.

(I also wonder how many kids will walk away from this series thinking that "pi" has that name because of a pie-eating party, that "diameter" is after Lady Di of Ameter, etc. etc. I'm not saying that the books shouldn't be read, but I wonder if literal minded kids will understand which aspects are made up since the math is all real.)