Not what you'd expect to see in a poem with so much "Fun," but what can we say, Billy rolls hard. In stanzas 6 and 7, things take a markedly dark turn. Instead of exploration and fun, we get… medieval. Not fun. It is also interesting that the images in the previous stanzas are all very free. There is freedom and movement inherent in exploration, adventure, and fun. All that movement is gone in the last two stanzas.
Lines 12-14: The poem is "tie[d] to a chair with rope" and "torture[d]." Doesn't get much more un-fun than that.
Lines 15-16: The final stanza leaves us with the image of someone being hit with a hose. These are really very dark images, especially in comparison with the fun images in the previous stanzas. Quite the contrast, don't you think?