Not on the strength of the family's status. A child in dependent status may study but is not employment-authorized, and an unmarried child under twenty-one is the limit of derivative eligibility.
The family business does not change dependent rules
A working family operation makes this restriction feel artificial, because there is obvious work and an obvious person to do it. The rule does not bend for family enterprises. A child in dependent status may attend school, including post-secondary, but may not be employed, whether by the family business or anyone else. If he is over twenty-one he is not a derivative at all and needs his own basis to be present. Where a son or daughter is genuinely intended to work in the business, that should be planned as a separate matter with its own route, and it should be planned early, because the alternatives take time to arrange and cannot be improvised after arrival.
Hypothetical example: A Canadian investor’s nineteen-year-old daughter expects to help in a United States specialty-food business after finishing school. The family must decide whether she will only study, seek her own work-authorized route, or postpone the role; dependent status does not authorize her employment. Her age, relationship evidence, and actual duties are the records that settle the immediate issue. Planning this before the business opens avoids building a staffing plan around a family member who cannot lawfully fill the position.
A business plan should use a lawful staffing alternative until that separate decision is resolved.