You asked:
Go to a place like Safe home.. on the web, or your local electronics shop, or search web for the stuff I will mention.
They have low light cameras that go to tape machines. They have IR activated lights and / or cameras. They have IR trail cams.. Seems to me that a group of neighbors could get some high tech stuff and secretly film where it is happening. For pretty cheap really..
If that is not liked or wanted: Call the Local DA’s office and your local elected officials as a group, or each call one after another.. Demand that they get caught. Nothing like pressure from the top down to provide incentive to the local police. IF enough pressure is exerted, a search warrant could be issued, and I am sure stuff would be found if they keep doing this.
As a group call the local TV and paper and complain… Nothing like having the TV and reporters about to get action or cause those doing it some discomfort.
If that is not liked: Get ALL the neighbors that do not like it.. together, and go over to the house, or houses, and ask to talk to them about this behavior, be friendly, be clam, but be frank about how this affects your life, and ask that they could stop it or limit it to certain times, etc. Possibly invite them to a cook out and get to be friends or know them, and then you can possibly get them to stop that way.
Finally: Call the FBI and / or the Dept of Homeland Defense, tell them you think they are making bombs over there… bet they would get caught then..
Or... live with it... This will pass, likely soon as the weather, changes and they get older.