There is due process. If they want to contest the no trespassing order, the kids can go to court to ask the court to issue an injunction against the city enforcing the ban. Be prepared to spend several thousand dollars in legal fees doing that. I cannot guarantee your kid would win that. Perhaps the court would uphold the ban but with some modifications to ensure that he can access city property for real needs, like going to court, etc., and limiting the ban to some specific period of time.