As others have said, lock the door to the trailer and your gate. Not doing so can invite more than just a couple of teens "hooking up" as they may begin to do other things in there as they become more comfortable. Another negative is that these types of places begin to be talked about and you might find others besides your neighbor's kids in there and that can lead to other problems.
In addition to locking the doors, I would contact the police on the matter. A squad car sitting in front of the house with an officer going back and looking things over can be a deterrent to further behavior. In any event, it establishes a continuing habit which might be helpful for investigation and/or insurance claims if needed.
Another possibility is what happened to the owners of the cabin next to ours. They were experiencing break ins to their cabin but nothing of substance other than food was taken. Evidence showed whomever was breaking in appeared to be sleeping there rather than stealing. It was deduced it was likely one of the teenagers living on the road. For this reason the owner did not contact the police.
A few moths later, the son of one of the permanent residents shot and killed his stepfather for abuse of him and his sister. Investigation resulted in similar suspicions in another jurisdiction but the investigation was dropped when the family moved.
If such is the case for you, locking the doors may not stop the abuse but involving the police may help uncover such actions.
