The substantial presence test does not apply to APs. They fall under one of the exemptions and therefore cannot count any of the time here under the AP program.

https://www.irs.gov/individuals/international-taxpayers/substantial-presence-test
Do not count days for which you are an exempt individual. The term "exempt individual" does not refer to someone exempt from U.S. tax, but to anyone in the following categories:
- A teacher or trainee temporarily present in the U.S. under a "J" or "Q" visa, who substantially complies with the requirements of the visa.

https://www.irs.gov/individuals/international-taxpayers/exempt-individuals-teachers-and-trainees
A teacher or trainee is an individual, other than a student, who is temporarily in the United States under a "J " or "Q " visa and substantially complies with the requirements of that nonimmigrant status. You are considered to have substantially complied with the requirements of that nonimmigrant status if you have not engaged in activities that are prohibited by U.S. immigration laws and could result in the loss of your nonimmigrant status. Any nonimmigrant temporarily present in the U.S. in "J" or "Q" status who is not a student is included within the definition of "Teacher or Trainee." For example, alien physicians, au pairs, short-term scholars, and summer camp workers temporarily present in the U.S. in "J" nonimmigrant status are included within the IRS definition of "Teacher or Trainee." In addition, cultural exchange visitors in "Q" nonimmigrant status are also included within the IRS definition of "Teacher or Trainee".

Check with your LCC. This is confusing. The IRS website says that au pairs are nonresident aliens:

https://www.irs.gov/individuals/international-taxpayers/au-pairs

So, it turns out au pairs may be able to collect $1,200 under the Cares Act:

https://www.wmdt.com/2020/04/j-1-visa-holders-eligible-for-stimulus-checks/

And here:

http://blog.sprintax.com/nonresident-aliens-guide-navigating-covid-19-cares-act-stimulus-payments/

“In short, the CARES Act can be claimed by US citizens, permanent residents and residents for tax purposes (individuals who can pass the Substantial Presence Test) who have a valid Social Security Number (SSN) and who have filed their 2018 tax return (in 2019), or their 2019 return (in 2020).”

Here is the key distinction: au pairs are not nonresident aliens, they are resident aliens in the U.S. for tax purposes if they pass the substantial presence test - which they most likely will, depending on when their au pair year ends in 2020. As a practical matter, they may not get a check mailed to them, unless they extended, are in their second year, and filed their taxes for 2019. They won’t be on the government’s radar. But it will likely reduce their tax liability by $1,200 as a tax credit, when they pay taxes for 2020 in 2021.

This all assumes they got a Social Security # and filed a tax return for 2019. If they were here in 2019, they can still file for 2019, as the date has been extended to July 15, 2020:

https://www.irs.gov/newsroom/tax-day-now-july-15-treasury-irs-extend-filing-deadline-and-federal-tax-payments-regardless-of-amount-owed

They can check the IRS website here:

https://www.irs.gov/coronavirus/get-my-payment

Ironic twist - “My income does not qualify my household for any stimulus funds, except my au pair.”