it's not because they don't say it at all.
It's because they think you're too humorless
to appreciate it.
I have heard parents and critics say that the books are valuable despite their poop references, anti-adult themes and obnoxious main characters. And I hear what they're saying--which boils down, basically, to "at least they're reading." The hope is, in a nutshell, that kids will read these during the years that many kids lose interest in books (the years after picture books but before books are assigned in school). Captain Underpants will at least keep kids reading books until they're old enough to discover the wonders of Mary Pope Osborne's Magic Treehouse and, as I have promoted here, Donald J. Sobol's Encyclopedia Brown.
But let me offer another view. Perhaps the Captain Underpants series are valuable precisely because of their poop references, anti-adult themes and obnoxious main characters. Here's why:
poop references: Little kids like to say "poop." It's a funny word that refers to a funny phenomenon. You say your kids don't (or didn't) use that word (and others relating to it) purely for comedic value? Sorry. If your kids don't say "poop" around you, it's not because they don't say it at all. It's because they think you're too humorless to appreciate it. Words can be funny. Words have power--to make people laugh, to make people cry, to make people angry. There are appropriate and inappropriate places to use certain words. Learning when and when not to say "poop," is a very small first step to learning about the power of words and how to use them.
anti-adult themes: Kids have very little power over their own lives. Don't want to go to bed? Tough. Don't like carrots? Too bad. Don't want to be physically removed from your friend's house and taken home for a nap? Doesn't matter. That's just the way childhood is. So imagine what it must be like to read about kids having ultimate power over their principal, turning him into an underpants-wearing superhero and back again! Amazing. Of course it's disrespectful. If it were possible to do it real life, it would be totally inappropriate and forbidden. But it's not possible to do in real life. It is, however, possible to imagine. Imagination is a totally appropriate way for kids to live out what they want to do but can't. In fact, they should be encouraged to do so. Keep in mind that in a little kid's everyday, real life, an adult can physically pick them up and remove them from one place and put them in another. So it's not all that hard to see why they'd like to read about that happening to a grown-up. And in his underwear to boot.
obnoxious main characters: George and Harold make trouble constantly, disrupt school, and mistreat their teachers and principal. Does any parent want his or her kids to act this way? Of course not. But what's wrong with reading about it? The events in the books are presented as outrageous and ridiculous. So, yes, the boys' behavior is outrageous and ridiculous. And kids love to read about things like that. About things they could never do, like fly or cast spells or start their own country. If your kids try to pull some George and Harold antics, tell them to stop.
The best thing about Captain Underpants books is that they teach kids that books (and adults who write them) can use the words "poop" and "tinkletrousers!" (now that's a funny word) Books can be about kids who do unacceptable things and outsmart adults. And adults can, like Pilkey, have a silly sense of humor and an endless imagination.
Books can be about anything. Anything. And they are a safe place to go when you wish things in everyday life were different, or just when you need a trip to an entirely new reality. I want my kids to learn that. That's why I want my kids to read Captain Underpants.