Based on the answer to this question https://ttlc.intuit.com/questions/3868340-filling-out-my-child-s-tax-return-tt-complains-about-negat..., I fixed the problem by redoing the interview for "Child's Income (Under Age 24)" in the "Less Common Income" section. In the page "Parents' Capital Gains", when entering a value for "Schedule D, Line 15", you should click on the "Learn More" link where there is a rather important note that If Schedule D, Line 15 is a loss, do not enter an amount on this line and leave the line blank.
