The siblings “started to dig a hole in the sand" on Montesilvano beach "convinced that, maybe, they would find pirates’ treasure”, TGCOM wrote.
Instead they found a "large, heavy" bag that contained "at least 2.5 kilos" of the world's most widely-used drug, La Repubblica reported.
The pair showed the bag to their parents, who alerted the lido’s manager.
Police then kept surveillance in a bid to catch somebody retrieving the bag, but noone came, La Repubblica said. They are now trying to get to the bottom of who was behind the hidden stash.
Earlier this year, Italy relaxed penalties for the sale and possession of cannabis in a bid to ease prison overcrowding. Several regional-level governments have also called for the drug to be decriminalized.
Cannabis is "by far the most widely cultivated, trafficked and abused illicit drug", according to the World Health Organisation.