For the Dependent Care Credit, Turbotax says: "Go ahead and add (my second dependent) with $0 expenses to increase your credit by $143." Is this correct?

The credit is so you can pay for care so that you (and your spouse if married) can work or go to school full time.  Do you even qualify, if you did not pay for care for the younger child?  Was placing your child in preschool necessary so you could go to work?   Even though you have a care expense, and you can get the credit on your tax return, you can be at risk if audited, unless the placed the child in preschool so you could work.

Assuming you are eligible, this error seems to be a mistake by Turbotax.  The IRS allows you to claim the credit even if you paid $0 for one of your qualifying children.  To get around this, try adding $1 to the second child and reducing the first child's expenses by $1.