Benefits continuity and immigration status are separate matters. A spouse and unmarried children under 21 may qualify for TD, through steps that depend on nationality. Keeping Canadian coverage does not create or replace TD, and each family member's status is assessed individually.
Settle the spouse's work question before the move
Secondments often assume the household relocates while the spouse keeps a Canadian role. TD permits study, but it does not authorize work, and that includes remote work for a Canadian employer performed while in the United States. Raise that with counsel early, since it can change whether the assignment is financially workable. A child who is a U.S. citizen does not need TD.
TD answers one question: whether studying is permitted while the principal holds TN status. Families sometimes read a benefits package covering dependants as confirmation of their immigration situation. The common misunderstanding is that a salary paid in Canada by a Canadian employer sits outside U.S. rules. If the spouse has an independent professional opportunity, assess a separate applicable classification on its own conditions. The usual mistake is assuming unpaid help, orientation, or remote services for the U.S. employer fall outside the work restriction.