I don't get the reference

The reference is to “Fenimore Cooper’s Literary Offenses” a takedown Mark Twain, considered by some still one of the most savage literary criticisms ever published, made about Fenimore Cooper’s book “The Pathfinder, or The Inland Sea” where among other offenses (literary and otherwise) a character proves a “good marksmanship” by hitting the exact same bullet hole as a first and then the main character proves to be even better by doing it a second time (a third bullet through “the exact same” bullet hole. (Among other things, multiple bullets through the “same” hole would expand the hole making it easier to hit, not harder. Which also relates to the fallacy of the given tests being additive. Presumably if the shot is lined up to be fair it is easier than the sum of its parts.)

Thank you :). And if people say it's *Mark Twain* at his most savage then I definitely should check it out, that sounds pretty brutal!

Also, your last point does make me wonder if this XKCD set-up is safer for the kid due to the axes blocking the "unsafe" path, or if it's more likely to make the arrows go off-course, making it unsafer instead.

Yeah, that's exactly my reading of it: the axes block most of the key "unsafe" paths for the kid. If the arrow makes it out of the axes with enough momentum it should be lined up that the only thing it can hit is the apple, and if that's the case the apple appears to be lined up so that just about the only thing it can hit is the bullseye (in a big splattery mess). But also the momentum has to be correct to even exit the axes (because the axes are aligned straight and not a parabola the only way to get through them is with enough speed/momentum to counter gravity at least that distance). The nature of that challenge is that if all the elements are set up to be fair and it is just a challenge of getting the strength/momentum right then just about the only options are you succeed or you spectacularly fail. Essentially the two additional tests are already solved by the first one, assuming the first one is aimed fair at the other two.