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Yes, I do want to be that neighbor at 26! Thanks!

I want people to stay off my property unless they're invited. I don't care if it's two girls checking out my dock, a man and his two teenage sons fishing, or whoever the person was who went around and knocked on all our windows in the middle of the night a few years ago. If you can't see the "wrong" factor in all of that, then maybe you need to evaluate yourself, not me. I was brought up to respect other people's property - not to trespass, not to run through the flower beds, not to take something that doesn't belong to me, etc.
And as far as the calling the police comment, I was referring to the previous poster who said I would now need to worry about retaliation. Would you not call the police if you were harrassed or your house was vandalized?
Well, good luck with that
Your "making it a point" is very likely to backfire on you and cause more trespassing than halting it.
I can just see it now. The kids whispering to each other - "you do it", "no, you do it" to see which one is the bravest to cross the witchy woman's property and to see if they can get her to come out and yell at them.
Every neighborhood has one of "those neighbors" and kids throughout history have been dared to cross the crabby neighbor as a right of passage. Heck, it is so common, scenes like this are in many kid movies; boys huddling, daring each other to step foot on the witchy neighbor's property. The woman comes out, disheveled, brandishing a broom, yelling at the monsters to get off her property. Boys laughing that they did such a brave thing, crossing the curmudgeon's property.
I would pick my battles. Boys playing football in your backyard - ask them to leave.
Stomping around in your flower gardens - ask them to leave.
Knocking on windows in the middle of the night - call police.
Trying to steal something - definitely call the police.
2 girls innocently daydreaming on your dock - chalk it up to childhood and enjoy watching children be able to still daydream without the pressures of adulthood. If they had packed a lunch and were spending the entire day, I would gently tell them that they needed to go home. But a few minutes, an hour - nope. They are not hurting anything.
If it really bothers you to have people cross your yard, put up a fence.
As the old saying goes: Fences make good neighbors.
FTR - my children were also taught to respect property. Hopefully they do 99.99% of the time (we all make mistakes). However, on my own property, life is not completely so black and white. Vandalism, destroying property, or blatant disregard gets called on. Childhood innocence, not so much.