Magic by the Lake (Tales of Magic #3)

If Jane and Mark and Katharine and Martha had stopped to think, they might have ordered magic by the pound, or by the day, but a lakeful of magic causes extraordinary and unexpected events involving mermaids, pirates, and cannibals. By the end of vacation, with an unwilling sly old turtle, and Ali Baba's forty thieves, they help stepfather Mr. Smith save his failing bookstore in a most surprising way.