First of all, not all beaches in Italy even have sand to build Castelli di Sabbia (sandcastles). Secondly, even if you were on an Italian beach with lots of fine sand and started to build a sandcastle with your child, more than likely a group of Italian children would gather around, never having seen anyone do such a thing. Thirdly, if you are on a beach like Eraclea (near Venice), the bagnino (lifeguard) would come over to stop you and give you a fine. It's apparently illegal, by mayoral degree, to build sandcastles as they block the public access.
Curiously, there are still some very large sandcastle competitions all over Italy during the summer months... one of the most prestigious being on the lido in Jesolo, just east of Venice, another in the seaside town of Cervia near Ravenna, and yet one more in Alessio on the Italian Riviera west of Genoa. Another unexpected one is on Easy Living's Urban Beach on the banks of the Arno River in Florence, a hipster beach bar.