In Pennsylvania, in the case of children who may be tempted to trespass, landowners could expose themselves to an “attractive nuisance” law suit if there is a structure or scenic resource deemed too attractive to expect no one to be tempted to use your lands. In some cases, people with swimming pools had to erect fences high enough to keep the neighborhood kids from pool hopping and possibly drowning. The general liability principle is as follows: The gravity of the harm in one hand, versus the likelihood of its occurrence plus the cost of prevention. If the gravity of harm is kids could fall down a well, and there is a school house nearby, then a fence may be in order. Be cautious, you only have the right to use reasonable force to protect your property, even in the case of fencing, etc. So don't go overboard with keeping people out either. In short, the law requires you to be reasonable, put people on notice and protect yourself from tempted, underage trespassers.