If the child who did not go to daycare could have qualified for the credit, the limit for your expenses is $6,000. To qualify, both children have to be claimed as dependents on your tax return, and both have to be under 13 years old. You enter both children for the credit, and enter $0 as the amount paid for the one who did not go to daycare.
IRS Publication 503 says "The maximum amount of work-related expenses you can take into account for purposes of the [child care] credit is $6,000 if you have two or more qualifying persons even if you only incurred expenses for just one of them."
