This was a Turbotax bug. Line 15 should be 0 in most cases when the child's income is primarily qualified dividends and capital gains and lines 18 and 19 of Schedule D are zero. This seems to be finally fixed in the latest updates (May 26) to the desktop software. I had to file for an extension with an overpayment and will now file for a refund of the bogus excess. Thank you so much Turbotax for delaying this until after the filing deadline while giving us no support for reporting this error. See this totally wasted Twitter thread:
https://twitter.com/capital_climate/status/[phone number removed]1208839
