"Please make sure you don't allow your children to run off alone to an area you haven't personally surveyed." <- a shame nobody ever says this about the world online...
A few years ago my son and his friend found a large knife semi-hidden in Fairland Park and decided to bring it home wrapped up in his school jumper.
I phoned 101 to explain what had happened and was told to take it to the nearest police station at some stage.
But the main piece of advice I was given was because he was 10 years old at the time, he was of an age to be charged with criminal responsibility and should have told an adult rather than bring the knife home. I explained this to him and his friends with the added warning of not touching injection type needles, guns or anything else they happened to come across which might be 'interesting' to curious children.