I put that my children were with me for 6 months and I paid more than half of their expenses but it won't let me add child care services. How do I fix that?

Have you entered any earned income from work on the tax return?  If married, did you enter your spouse's earned income from work on the tax return?

Have you entered your income? In order to enter child care expenses you have to have income from working - either a W-2 or self-employment.

Also, the tag below your question says you are using TurboTax Free Edition. You cannot claim the child care credit in Free Edition. You will have to upgrade to Deluxe.

Have you entered your W-2 or self-employment income yet?   The childcare credit will not work unless you have entered your income.    What filing status are you using?  If you are filing a joint return you must show income from each spouse.   If you are filing married filing separately, you are not able to use the childcare credit.  

The children must live in your home more than half the year (183 or more days).  Unfortunately, Turbotax interprets the choice "6 months" to be exactly half the year, and half the year is not more than half the year.  If the children did live in your home more than half (6 months plus 1 day, or more) change your answer to "7 months."