The IRS requires their addresses, Social Security or employer identification numbers, and all payments they receive from you to be reported in order to claim the dependent care credit.
If the nanny refuses to give you the identifying information, you should report on Form 2441 whatever information you have (such as the name and address). Enter “See Attached Statement” in the columns calling for the information you do not have. Then attach a statement explaining that you requested the information from the care provider, but the provider did not give you the information. Be sure to write your name and social security number on this statement. The statement will show that you used due diligence in trying to furnish the necessary information.
https://www.irs.gov/publications/p503/ar02.html
[Edited: 1-23-18|2:20PM]
