It is indeed a young adult book. But she's only nine so of course it can't be the same level of sophistication as Nation or Amazing Maurice. It continues (more or less) Lord and Ladies, and feels really like an intro to Tiffany starting her witch career. But she is a Witch, just like Agnes has to be a Witch too. The witchcraft chooses you... (?) It feels very Narnia, and at some points Alice in Wonderland.

I ... I didn't start Discworld expecting it to be so much pain. How could he put so much Aching on a nine-year-old (?). But Pterry did write every one (except the wizards, whom I assume he didn't like either)(apart from Ponder and maybe Rincewind) with their own pain in life, their struggles, darkness, doubts, and ... and then there is Thief of Time, and there is Last Hero (well Last Hero is not very living-life, but just a little not-dying).