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Your kid does not need to get the TN visa and just bring the TN required documents and show these documents and the Candian passport to the immigration officer at the port of entry to get the TN I-94 to enter US to work. If the officer asks about the immigrant visa application, just tell the officer that your kid will return to Canada to apply for the immigrant visa later. If the officer does not ask about the immigrant visa, then do not mention it.

Talk to the employer and the lawyer about the TN application. See this link about the TN application:

TN NAFTA Professionals | USCIS