Earwig and the Witch by Diana Wynne Jones

Earwig's lived in the orphanage as long as she can remember.  And she likes it there - not only does she have all the other kids doing what she wants, but the grown-ups, too!  In fact, she likes living at the orphanage so much, it's her plan to never leave.  She accomplishes this by making herself as undesirable as possible when prospective parents come to look at the children.

But one day, a very odd couple come in, and - despite her best efforts - adopt Earwig.  With no other choice, Earwig goes home with her new parents.  But the strange couple turn out to be the witch Bella Yaga, and The Mandrake (Jones never explains what a Mandrake is, but it's clear he's one bad, bad demon).  Earwig has been adopted to be Bella Yaga's slave, helping her prepare the ingredients for her magical potions.

Determined to figure out a way out of her predicament - after all, Earwig is used to telling others what to do, not being told! - Earwig finds opportunities to learn some magic of her own, with the help of Bella Yaga's reluctant familiar, a talking cat named Thomas.  A fun early chapter book.