Rufus and the Witch's Drudge - Marilyn Levinson

Fifth-grader Rufus Breckenridge is spending the month of July in the South of France with his best friend Billy Cameron and Billy's parents. They are staying at a villa owned by the Beaufonts, friends of the Camerons. Rufus has a crush on Danielle Beaufont and hopes Billy won't slip and tell her he's a witch because she's sure to think he's weird. 

While down at the beach, the boys spot an old woman whom Rufus knows is a witch. They follow her and discover she's always accompanied by a young girl whom she treats as her drudge. Danielle arrives and discovers what the boys are up to. She thinks it's cool that Rufus has magical powers. She tells them the old witch's name is Grendel, and she's the last of a rich and powerful family of witches. Grendel has taken Violette from her family because the little girl has great magical powers. 

The children realize that Grendel, whose own powers are fading, is using Violette's magic to steal art and other items from local villas that once belonged to her family. And then, just before the DeGama celebration, DeGama's famous painting "La Gitana" is stolen from the local museum. Can Rufus, Billy, and Danielle return the painting, defeat Grendel, and return Violette safely home to her family?