Per IRS Instructions for Form 2441 Child and Dependent Care Expenses, on page 1:
Married persons filing separately checkbox.
Generally, married persons must file a joint return to claim the credit. If you claim the credit and your filing status is married filing separately, you are required to show you meet the special requirements listed in sections, Married Persons Filing Separately and Who Can Take the Credit or Exclude Dependent Care Benefits, by checking the checkbox located above Part I on Form 2441. See Married Persons Filing Separately Checkbox, for more information.
On page 2:
Married Persons Filing Separately
Generally, married persons must file a joint return to claim the credit. If your filing status is married filing separately and all of the following apply, you are considered unmarried for purposes of claiming the credit on Form 2441.
If you meet all of the requirements to be treated as unmarried and meet items 2 through 5 listed earlier, you can generally take the credit or the exclusion. If you don't meet all of the requirements to be treated as unmarried, you can’t generally take the credit. However, you can generally take the exclusion if you meet items 2 through 5.
For additional information, please refer to IRS Publication 503 Child and Dependent Care Expenses.
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.