I know that the Tiffany books are called "children's" but I think that's somewhat misleading. It's true that the protagonist is a nine year old girl in the first of the series--but she is a nine year old untrained witch which is a different kettle of fish altogether.
And this series is definitely one where the character develops--as she ages, she begins to have more and different problems. I can't wait for Tony to get here with my copy of I Shall Wear Midnight. So, I can't have a problem with spoilers for that one--but these books deal with human problems and themes, some of which affect children also. WFM, for example, deals with grief/loss, learning to take responsibility for one's actions and a fairly sophisticated examination of the responsibilities which each person has, whether young, old, rich, poor, powerful or voiceless.
I have found that, as with many of Terry's other books, one needs to read them in order, but also more than once because (especially with the later ones) they become more complex and deal with bigger issues.
So rush right out and get all of them. I really like these (as a whole) as well or better than the regular witch books (as a group) because I find the earlier witch books less interesting and less well-written.