You don't necessarily get a refund of the other half of the child tax credit. It's a credit that is applied against your tax for the year. The amount of the credit that you are getting is on Form 1040 line 28.
The reason that your refund decreased is that TurboTax started off giving you the full credit for the entire year. When you entered the amount that you already received, it subtracted that so that you don't get it twice.