I have very fond memories of Robin Jarvis
, who wrote the Deptford Mice series and The Whitby Witches, amongst others. I was also a huge fan of The Borrible trilogy
^ by Michael de Larrabeiti (I still am, in fact.). Although I wasn't a child when I found them, Neil Gaiman's YA work is astonishingly good. I love them, as does my fantasy novel hating wife, my father and my Doctor Who obsessed 13 year old nephew. Diana Wynne Jones, is always worth reading as well, however old a child you are.
I don't know if they were originally written with younger readers in mind, but I would heartily recommend the Firekeeper series by Jane Lindskold and of course, the Earthsea novels by Ursula K. Le Guin. Robert Westall
wrote incerdibly good children's books as well.
^One word of warning, even thirty three years after first publication The Borribles are still quite controversial in a lot of parents' eyes.