Please explain if you are asking about a self-employed worker or a W-2 employee. If you are asking about a W-2 employee, then mileage is not deductible on your federal return. Your hypothetical nanny could be a W-2 employee. The new tax law that went into effect for 2018 returns eliminated job-related expenses as a deduction for W-2 employees.
https://ttlc.intuit.com/questions/4482873-which-federal-tax-deductions-have-been-suspended-by-tax-re...
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