I will skip any spoilers, because I want you to read this book.
Our heroine is Molly, twelve years old, who wants to be a wicked witch. (She has a good twin sister, so it makes sense that she's the evil one.) When she finds a place that needs a new evil master, Castle Hangnail, she jumps at the chance, even if she does a break a rule or two (or some other uncomfortably high number that's much larger than two). Meanwhile, the minions of Castle Hangnail have been desperate to get a new master, and if they don't, the castle will be forced to close. (This isn't a pleasant thing either for the minions or the castle.)
The tone of the work is light and fun, yet consistently sincere. There's not a hint if irony or sarcasm or pessimism, except where useful. The plot moves along jauntily (I don't believe that I used that word), always spending enough time with the characters, yet never so much time that the jokes grow dull or the scenes drag. All the minions are firmly realized, getting their moments to shine. And you even get some gardening tips as well.
If you read aloud to your kids, I doubly recommend this book.