Yes, harassment prevention training is designed to inform employees about what constitutes harassment and abusive conduct, how and where to report it, company policy and the obligation to investigate, correct and take disciplinary action for violations. An employee who is a minor (under 18) must be included in that training.
There is no exemption in the law that would exclude any employee for any protected class basis (such as religion, sex, national origin or age). Neither is there any exemption for employees who are part-time, temporary or seasonal.
Training is required to protect employees from being subjected to such conduct. Certainly, a company would not want to exclude minors from such training, which may leave them more vulnerable because they would not know what to do about the harassment.
As with all matters concerning minors, the parent or guardian should be notified that this training is required by state law for all employees and asked if they want to schedule a time when they could be present for the training. That should satisfy any concerns that a parent may have about what their child is being told about harassment and abusive conduct.
Read more about Mandatory Harassment Prevention Training in the HR Library.