While working in Canada I had a boss who was a US citizen, but he had been born in Canada to US married parents. He had the tax id for his parents to claim him as a dependent till 18. But he did not have a SSN number. He refused to work in the states because he didn't want to get a SSN number and thus have to pay taxes for the rest of his life, but he was still a US citizen. I have no clue if that was legal or not. And I have no idea if this matches your circumstances, but it may be something you want to look into. See if they will be forced to pay taxes even if they don't have an SSN number just the tax id (which is different for children, or so I've been told).
I am shocked that someone posted something potentially informative.