Thanks, Chris. I figured Discworld would be a good topic for making a test post on this platform. It seems pretty good so far.
The quantity and quality of Pratchett's writing is incredible. He wrote about two books per year up until his death a few years back, which makes waiting upwards of 6 years for a new book from certain other authors (cough RRMartin cough Rothfuss) seem a little ridiculous.
Pratchett's young adult books are just as good as his other books, with some people even considering them the crowning achievement of his series. If your daughter likes Harry Potter, she will love Tiffany Aching. In the first book in her series, The Wee Free Men, Tiffany is 9 years old and decides to become a witch after seeing an injustice done to an old woman, and then has to save her brother who was kidnapped by the Queen of the Fairies. The subsequent books follow her as she grows up and learns more about witchcraft, so it's similar to Harry Potter in that respect. It's a great series with plenty of strong female characters. It should be just as enjoyable for a 12-year-old boy, if he's not too cool for a story with a girl as the lead. Maurice and his Educated Rodents is his other YA book, but doesn't have any direct sequels. Otherwise, one of the other story arcs might be a good place for him to start. The City Watch arc, starting with Guards! Guards!, is a great crime thriller series.
None of his books ever go above PG-13 type content, so they're all safe for young readers. That being said, Tiffany's books are very earthy stories, with Tiffany growing up on a sheep farm, so there are subjects like lambing and small hands being useful during difficult births mentioned in passing.