The spouse and unmarried children under twenty-one may accompany in dependent status. They may study but may not work, and derivative status ends at twenty-one for each child.
Decide who applies before anyone resigns
Hypothetical example: the accountant's wife teaches at a rural school and their son is fifteen. She cannot work in the United States as a dependant of a TN holder, and teaching is regulated in every state, so even a route of her own would carry a licensing process of its own. If both adults intend to work, the household should decide which of them has the stronger independent route before either resigns anything, rather than assuming the offer in hand settles the question. Their son may attend school and may not take a job, and his derivative status ends on his twenty-first birthday regardless of what he is studying. Write the household out as a list with dates of birth, statuses and intentions, and revisit that page before every extension.
Hypothetical example: the principal applicant's spouse is a self-employed bookbinder who expects to keep taking Canadian commissions after the move. The household's first review should produce a separate activity plan for every family member, including where each person will physically perform work and whether an independent category is needed. The passport, marriage certificate, and each child's birth record settle the relationship evidence; they do not create work permission. Put school enrollment, leases, and employment decisions after the status choice, not before it. A frequent mistake is assuming that invoicing a Canadian customer makes work permissible from the United States. The relevant issue is the activity carried out there, so the family should obtain advice on that fact pattern before relying on derivative status.