Unmarried children under twenty-one may qualify for TD, which permits them to attend school. Age and marital status matter individually, so a child approaching twenty-one changes the planning horizon for that child regardless of the parents' arrangements.
Track each person's own dates
It is easy to treat the household as one case. In practice each dependent has their own status, their own validity period, and in a child's case an age limit that arrives on a fixed date. Note those dates alongside the principal's period and the academic calendar, so that a program spanning a birthday does not create a gap nobody planned for.
Build a dated review file around each dependent's passport, admission record, daily activity, school plans, and proposed paid work. TD classification permits a spouse and qualifying children to study, but it does not authorize them to work, freelance, invoice through a Canadian company, or provide unpaid labour that is really employment. TD status allows a spouse or child to accompany and study, but it does not authorize employment, including remote services performed while physically in the United States. TD status permits study but never authorizes employment, so translating brochures, answering customer messages, or invoicing through a Canadian account can still amount to work rather than harmless family help.