My wife and I paid for daycare for both of our children in all 12 months of 2020, yet for some reason can only claim Child and Dependent Care Credit for the oldest child?

[kkessing2016]

Oldest child (3 years old) is eligible for the Child and Dependent Care Credit. Youngest Child (19 Months old) is not. No explanation provided by TurboTax fit our situation.

[KrisD15]

Did you AND you wife each earn enough to take the credit for both children? 

According to the IRS:

"the expenses claimed may not exceed the smaller of your earned income or your spouse's earned income;"

Topic 602

[Opus 17]

To be eligible, the child must be under age 13, have a valid social security number, and live with you more than half the year (in the case of parents who are separated or divorced).  Review the child's information, paying attention to those items.