No schedule H is needed unless the Au pair has been in the US previously or is considered a resident alien.
From the link I provided earlier:
Social Security and Medicare Taxes
Au pair wages are not usually subject to social security and Medicare taxes because of the au pair's status as a J-1 nonimmigrant and as a nonresident alien. However, if the au pair had previously been in the United States as a student, teacher, trainee, or researcher in F, J, M, or Q nonimmigrant status, then the au pair might be a resident alien during their current stay in the United States, and might be subject to social security and Medicare taxes if their annual au pair wages exceed the applicable dollar threshold found in Publication 926. Please refer to Alien Liability for Social Security and Medicare Tax.
If the au pair is a resident alien and their annual au pair wages exceed the applicable dollar threshold, then the host family must withhold social security and Medicare taxes and report them on Schedule H, Household Employment Taxes, of Form 1040 and on Form W-2. The host family will need to apply for an Employer Identification Number (EIN) if it is required to withhold tax and file Form W-2. The au pair will need to apply for a U.S. Social Security number.
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