Per IRS Instructions for Form 2441 Child and Dependent Care Expenses, 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 now 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.

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.

• You lived apart from your spouse during the last 6 months of 2018.
• Your home was the qualifying person's main home for more than half of 2018.
• You paid more than half of the cost of keeping up that home for 2018.

If you meet all the requirements to be treated as unmarried and meet items 2 through 5 listed earlier, you can take the credit or the exclusion. If you don't meet all the requirements to be treated as unmarried, you can't take the credit. However, you can take the exclusion if you meet items 2 through 5.

For additional information, please refer to IRS Publication 503 Child and Dependent Care Expenses.