Real Estate Law
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I have no idea why law enforcement are telling you that these people can retrieve their tennis balls without your permission. In Illinois this would be considered a civil matter rather than a criminal matter.
The issue is that trespassing can be considered either criminal or civil matter. Since this involves neighboring properties the police are treating this as a civil matter. Who owns the tennis courts?
Depending on what was posted you may have a defamation of character lawsuit against the mother. What was posted?
You have a civil action against the owner of the tennis courts. You may also have a defamation of character depending on what was posted on FB. It would be my pleasure to provide you with a link for local attorneys that provide free consultations if you would like to pursue either one of these of legal matters.