You can always call Children's Aid and let them know that the kids are unsupervised and you are worried about their well being. Personally I would just call the cops and explain what you are going through. You have tried to be nice. I would also call the landlord and let him know you are going to call the cops and report the other tenant and that you are also going to be asking them to talk to him. Tell him that yo uhave voiced your complaint several times to him and he has not done anything and is not willing to and you have no choice but to involve the police. I would also findout who the other kids are and where they live and talk to their parents. No point dealing with them, they don't care. I've had similar problems and have even gone to their school and spoken to the principal and the school has taken care of some problems. The friends are the problem so talk to the kid in your building when they aren't around and he'll more than likely tell you where they live. Tell him you are calling the police and childrens aid if it continues.

The other option that has worked for me in the past as well is to get them involved and interested in what you are doing. If it becomes their project too they will want it to succeed. Good luck. Been there and I know it's no fun.