I have searched NC labor law and child labor law and could find nothing forbidding it because she was not working. I searched OSHA and could find nothing forbidding it because she was not working.
I am confident that it violates company policy. The employer may have liability if she were injured on their property; thus, the employer has a strong interest in keeping children and other people out of their property unless they are employees.
Thus, it appears that your son-in-law violated company policy, but I could find no child labor law or OSHA standard that he violated since she was not working but merely on the premises.
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