Can a non-compete clause stop a seller's adult child from working for a competitor?

Generally no, if the adult child is simply taking a job with a competitor on their own account. A restrictive covenant binds the party who actually signed it — ordinarily the selling shareholder — and Ontario contract law does not usually extend that obligation to a separate adult who was never a party to the agreement, even a close family member, based on the basic principle that a contract binds only the people who made it.

The analysis can shift if the child's employment is really a vehicle for the seller's own continued involvement in the competing business — for example, if the seller is directing the child's work, has a financial stake in it, or is using the arrangement to keep working in the business in substance. Many non-compete clauses are drafted broadly enough, using "directly or indirectly" language, to capture that kind of situation even though the covenant is not being enforced against the child personally, but against the seller for causing or benefiting from the breach. Whether a specific arrangement crosses that line depends on the facts and the clause's exact wording.