I can't read Pratchett myself. But isn't the point that he treats traditional fantasy elements for purposes of comedy? Isn't giving kids Pratchett before they have had plentiful experience of the real thing, like having kids watch fairy tale parodies on Sesame Street? -- another thing I despise. Poor little beggars, there's precious little in our foul culture to nourish their imaginations; we've got to ferret out the last remnants and hasten to draw moustaches on the stone lions for them? Fooey. Of all things one of the worst is to make youngsters "knowing" and "sophisticated" before their time, poor kids.
No. In
The Colour of Magic, the first novel in the series, he uses a number of fantasy novels as source material for jokes, but they're not the usual suspects (Tolkien, Lewis etc). Instead he satirises Lovecraft, McCaffrey (the entire third "episode" is based on
Dragonriders of Pern), Fritz Leiber (a lot) and Jack Vance. He doesn't expect everyone to have read those books so he makes the stories work even if you have zero knowledge of the source material, and in 2021 I'm not sure how many people, even older readers, reading fantasy have gone back as far as McCaffrey, Leiber and Vance given the quality of authors around today.
After that point he stops using fantasy as a source material (apart from the odd gag about inexplicable chainmail bikini), noting that would be lazy, and instead starts using everyday life as inspiration.
Moving Pictures satirises Hollywood,
Guards! Guards! police procedurals,
Jingo war films etc. The books also aren't necessarily about being funny, but move much more towards a more serious treatment of issues, even angry ones.
Small Gods eviscerates religious fundamentalism and the evil it drives humanity to do in a more precise and surprisingly darker tone than almost any other novel that comments on the phenomenon (there are still jokes, but you can tell there's a barely-suppressed, intelligent rage propelling the book).
Night Watch is one of the very few prequels that actually informs character and world development in a constructive and intelligent way.
Pratchett is one of the very best fantasy writers of all time and wrote some novels explicitly for children (the
Truckers trilogy, the
Johnny Maxwell trilogy,
The Carpet People,
Nation and
Dodger, all outside the Discworld setting, and
The Amazing Maurice and His Educated Rodents and the
Tiffany Aching books within it). If you want to give younger readers a grounding in fantasy, the question is more why you
wouldn't start them off with Pratchett, at least his children's books.