Do they qualify?  There are income limitations and age limits for kids.

Enter what was received for each Recovery Credit in screen 38.2. If one was not received, enter -1 for that payment.  This worked for the one return I tried. Remember that the amount is dependent on the 2020 income. 

I did that and it didn't work for me. I am lucky that I also use Drake, and it worked fine on Drake tax software.