I have enjoyed reading several Pratchett novels but this little "fairytale for all ages" was not one of them. Early on I thought first- or second-graders would giggle shamelessly if their teacher were to read this to them but closer to the end I realized they would be as bored as I was.
There were a lots of vocabulary words to talk about poo or going poo, such as doing one's business. And I suppose there was even occasion for learning new things, such as hippos whirling their tail around so as to splatter the poo as far as they can to mark their territory. This would be especially memorable if the children could watch a video in science showing such a thing.
That said, I would suggest not bothering with the book. Talk to your husband, children, or grandchildren to learn what there is to know about poo. It dawned on me as I wrote that last sentence that Pratchett is from England so maybe the husbands, children, and grandchildren over there don't talk about these things in polite company. Thus, the need for this book.