I found the TurboTax advice here to be a bit misleading.
To make this work, fill out the other child's return using the pre-TCJA option. Ignore the Tax Due amount (which in my case was thousands more than my child even had as total income), and go to the Form 8615 Line 5 worksheet. The amounts will line up with the requested information for the first child. Once you capture/print that information, change the TCJA option back (the worksheet will disappear at that point).
TurboTax needs to make this clearer, but in the meantime I was able to make this work, and the IRS did accept my returns.
