Magician's Ward — Patricia C. Wrede

When Mairelon made Kim his ward, he promised to teach her to be a lady and a magician. But magic proves to be harder than it looks, and being a lady is even harder. Before frustration--and Merrill's formidably correct aunt-- can drive Kim mad, a mysterious gentleman attempts to burgle the Merrill town house. Soon, disaster strikes Mairelon, and Kim braves London society and her old, familiar back slums to save him, helped by a London moneylender, a Russian wizard prince, seven legendary French wizards...and Mairelon's charmingly eccentric mother.