The son had already written the first two letters of his name using a small stone on a brick wall dating back to the ninth century, while his father was about to add his name, using a stick, before the pair was caught by staff and reported to the police, Articolo Tre reported.
The father attempted to cross out his writing, but was not quick enough to avoid being taken to the police station, where he was later charged with vandalism.
The 12-year-old was reported to the public prosecutor of the juvenile court for "contaminating a property of historical interest".
The stone was also confiscated and will be examined by experts to determine whether it is of any historical significance.
Italy takes protection of its monuments seriously, with last summer seeing a number of fines handed out for similar offences, including a child who wrote their name on Florence's Ponte Vecchio, and an American who snapped the finger off a 600-year-old statue in a museum in Florence. READ MORE HERE: 'Fingerless' statue culprit was a US surgeon