I came across this same issue. After a few hours trying to figure this out, I think Turbo Tax does not factor in qualified dividends as long term capital gain (ie. 20%) when it calculates family's aggregate tax (parents + child income). Instead, it treats qualified dividends as standard income and tax at marginal tax rate.
You can see this on form 8615D Wks. Page 2 is Family's Aggregate Line 7 Schedule D Tax Calculation. Line 2 on this page is for qualified dividends. However, it does not show the amount of qualified dividends that I entered. It keeps showing 0.
