To make a long story short,
Pub 929 outlines when to use the various tax calculation methods referenced in Form 8615:
Line 15 (Tax on Child's Taxable Income in Excess of Net Unearned Income)
Figure the tax on the amount on line 14 using the Tax Table, the Tax Computation Worksheet, the Qualified Dividends and Capital Gain Tax Worksheet, the Schedule D Tax Worksheet, or Schedule J (Form 1040), as follows.
If line 14 doesn’t include any net capital gain or qualified dividends, use the Tax Table or Tax Computation Worksheet (or Schedule J, if applicable) to figure this tax.
If line 14 includes any net capital gain or qualified dividends, use the Qualified Dividends and Capital Gain Tax Worksheet to figure this tax. For details, see the instructions for Form 8615, line 15.
However, if the child has 28% rate gain or unrecaptured section 1250 gain, use the Schedule D Tax Worksheet. (But use Schedule J instead, if it applies.)
There are no 28% rate gain or uncaptured section 1250 gains involved in either the child's or parents' return (Schedule D, Lines 18 & 19). However, TT is forcing the use of the Schedule D Tax Worksheet.
In contrast to the Schedule D Tax Worksheet's $100+ tax, the return's Qualified Dividends and Capital Gain Tax Worksheet calculates a $0 tax liability (Line 25) for the $1,101 income.
Unfortunately, the child has no farming or fishing income, so Schedule J is out.
FiveK, thanks again for pointing me in the right direction to dig into this!