Jan. you should know that girls don't exactly decide they are witches--the scenes in
L&L with Diamanda and with Agnes/Perdita in
Maskerade at least strongly suggest that witches don't start off with real magic (although Perdita's ability to sing with that range suggests supernatural power). You really must take time to read the Tiffany series. They connect so much with the other books of the Witch series. I was re-reading
Wintersmith while I was waiting to be called for X-rays today and hit a passage that resonates strongly with
Witches Abroad and the importance of stories. And that's just one example.
Tiffany, am I right in assuming from your comment that you have only read (or listened to)
WFM? I'm a bit confused as to whether you've read
HFOS. But if you think about it, Tiffany actually uses more headology (although to a great degree on herself) in
WFM than actual magic. In fact, one could certainly argue that what she does in this book is learn to use her intelligence to defeat the Queen's magic. Certainly she uses a form of headology in the battles with the Queen, and indeed that is how she defeats her--because Tiffany has gotten centered in herself and thus is able to confront and defeat the Queen in spite of her efforts to frighten Tiffany with her magical appearances.
Tiffany is, of course, a very unusual young witch and much more powerful (potentially) than the others of the young witch coven in the later books. And you become aware, in these books, of the dangers of an untrained witch which is why Granny takes such an immediate interest in her in WFM. Granny has discovered a young potential witch who has not only rescued herself but has saved her world from invasion--armed with what she has unconsciously learned from Granny Aching and a frying pan. She has learned from Granny Aching about being the hag o' the hills, and about responsibility. She has to learn to think about the world and observe or notice things--prime characteristics of a witch.
But she has a great deal more to learn about responsibility and self-awareness--something that is developed much more in the 2nd & 3rd books. The most striking thing about the Tiffany series, I think, is how much there is in them. I've had to read all of them multiple times to get to understand, I think, what Terry is doing and saying. It is possible to read them at a surface level (as is true with all Pratchett books) and I wonder exactly how much the younger readers get from them. I do hope that Terry gets around to writing Midnight soon, but unless the character of Esk has changed dramatically, I doubt that she and Tiffany will have much in common. After all--Esk is a wizard not a witch, and there's all the difference in the world (as Granny frequently says) between the two.
Changing thoughts--of the books that are usually listed as "children's"--which ones do you all think are really for younger readers. My own feeling is that although there is always more to Pratchett's books than appears on the surface, that the
Bromliead trilogy and the
Johnny books are the simplest, followed oddly enough by
Maurice which looks like a children's story but is one only in the same way that
Grimm's Fairy Tales are children's stories. Then the Tiffany books reach a new level of sophistication.
And BTW, Tiffany--Many Happy Returns of your Birthday! 
Sorry I'm a bit late with my good wishes. 
