It is worth holding firm commitments until it is known whether the position can support a TN route at all. Dependent TD steps follow the principal's case, and this question can end with the role needing to change or a different route being considered.
Keep the household decisions reversible
A spouse and unmarried children under 21 may qualify for TD, through steps that differ by nationality, and TD permits study but does not authorize work of any kind, including remote work for a Canadian employer. Where a household needs a second income, that plan should be built separately rather than around this offer's outcome. Until the duty question is answered, treat schooling, housing and notice periods as provisional.
If the child intends to remain for studies while the family’s plans change, confirm the lawful basis for that stay independently. Give counsel the actual program requirements, including unpaid placements and practical activities, before the dependent commits. The principal must be a Canadian or Mexican citizen with prearranged work in a listed USMCA profession and the profession's required qualifications; Canadian permanent residence is not citizenship. The common mistake is treating family assistance as informal and never testing it against the spouse’s actual immigration status.