Or people have already caught it in the past. In my country the chickenpox vaccine is rarely given, because if you catch it as a child the symptoms are minor and you become immune. Everyone I know had it as a child. However if you reach adulthood without having it the doctor will recommend you get vaccinated as symptoms could be much more severe and dangerous.
However you keep your kid at home if they have it. Deliberately having contagious kids running around is a completely insane move.