Good catch - it depends - for example, the parent’s rate on qualified dividends and capital gain income is 0% up to the combined income with the child of $74,900 (where the MFJ marginal rate of 15% ends).
So if MFJ parents had taxable income of $70,000, the child could have qualified dividends and capital gain income of $7k ($2,100 + $4,900) and would have a zero tax liability.
Again - none of the financial gymnastics and time/money spent with a CPA on tax projections may matter if the child would not get additional need-based aid.