The Roman Mysteries: The Pirates of Pompeii: Book 3 by Caroline Lawrence
It is AD 79 and Mount Vesuvius has erupted, destroying Pompeii. Among the thousands of people huddled in refugee camps along the bay of Naples are Flavia Gemina and her friends, Jonathan the Jewish boy, Nubia the African slave-girl, and Lupus the mute beggar boy. When the friends discover that children are being kidnapped from the camps, they start to investigate and soon solve the mystery of the pirates of Pompeii.
A terrifically exciting and dramatic story packed with superb historical detail.