Police are urging parents to talk to their children about the dangers of playing on building sites.
The warning comes after damage was caused to materials left on a building site at the bottom of Henthorn Road in Clitheroe.
PCSO Natasha Harris said: "Please can we ask that you advise your children of the obvious dangers of playing on building sites.
"Yesterday I attended an incident where several soft landing bags have been damaged at the building site on the bottom of Henthorn Road, along with several breeze blocks which have been thrown from a height and smashed.
"Soft landing bags are used to prevent harm to a builder working at heights in the event of a fall and they are a necessity, meaning that a possible knock on effect is that building work could be postponed if any more of these bags are damaged."
She added: "The most worrying thing for me was that these bags have clearly been cut with a knife. Also worrying is that these bags are filled with small polystyrene pieces and there is a stream which runs just behind the building site, meaning that this polystyrene could easily be blown in to the stream causing pollution and harm to our wildlife. As for the dangers of climbing on piles of breeze blocks and throwing them from a height - I think the dangers speak for themselves!"
Following the incident, the site's supervisors will be stepping up their own security and the police will also be paying more attention to the site.
PCSO Harris said she will also be contacting several parents and the schools of the children who the police caught on site last night.
Anyone with information is asked to call the police on 101 and quote log number LC-20170717-0307.