If the parents of these children won't do anything and neither will the landlord then I would take the matter to the police as surely this would come under invasion of privacy/harrassment. If the kids are coming into your property this may be seen as trespassing. I would keep a diary of events...
I had been letting a room to a friend of mine for 3 years and got on well until her brother came to stay for a while. i drew up a lodging agreement but after a few months i gave him notice to leave as he was very aggressive and intimidating. in short, i was scared to come home in case of his...