If you hired the au pair through an agency and paid the agency, then the au pair is not your employee and you do not pay household employee taxes or report the payment in any way on your tax return.
If you are trying to claim the child and dependent care credit on form 2441 using pre-paid au pair cost as your child care expense, then you must allocate the expense to when the care actually occurred. This is different from the normal rules of accrual and cash basis accounting, see IRS publication 503. In other words, you should have reported half the expense as a child care expenses on your 2019 tax return and half the expense on your 2020 tax return. However, you must have a valid Social Security number or tax identification number for the caregiver in order to claim the credit, and the caregiver would have to apply for an ITIN before you could claim the credit.
I can’t think of any other reason why you would be thinking of reporting this payment on your tax return, if it is not for the dependent care credit, please reply with more details.
