I like the thorn plan and sprinklers. We have a dog that digs massive holes. We just cover the area we want her to stop digging in with honey locust thorns. If you have ever been exposed to them you will never attempt to cross them again. They are like 5 inches long on a good specimen and branched. We just pull them off the tree in clusters and sprinkle them around. I would suggest a wide border.
I have children and have never had anyone complain about their behavior. Of course they know the consequences if I do get a bad report. They also don't leave my sight unless we are at home and they are playing outside. Keeping up with them kind of defines "Parenting", don't you think. We have a large pond/lake and have had people come to the house to buy chickens. The first thing they do is let their kids run out to the pond and get up on the deck. The pond is 12 feet deep in the center and is a mud bottom. I don't swim well enough to rescue them and forbid my kids to play around the pond. I feel your pain on unsupervised children. Unfortunately I would lose a law suit according to my insurance agent, which makes no sense to me what-so-ever.
I am not going to fence the front of my property. I worked too long to clean it up and have a nice level yard with the hammock strung and the gazebo up to enjoy my lake without ruining the view with a fence. I have a sign up that reads "All trespassers will be shot and survivors will be shot again" and I tell all people coming out that we are using biosecurity and to stay on the gravel path and boardwalks. I also tell them we have a guard dog, even though we don't and to keep the children with them. A bit excessive I know but better that than a law suit.