The kiddie tax law was originally established to prevent well-off families from abusing the strategy of shifting unearned income to their children, where it would be taxed at a lower tax rate than their own.

FORBES: The New Kiddie Tax

Individual income taxes as well as corporate taxes are now far more rooted in the shifting sands of volatile business income and capital profits rather than in the terra firma of wage income that stabilizes payroll taxes.

WSJ: Michael Solon: The Revenue Deficit From Progressive Tax Rates

Back in the heyday of using trusts for shifting income, if a trust contained one of those powers, like the power of the grantor to withdraw property from the trust and replace if with property of equal value, someone who thought the trust was being used to shift income, would consider that power to be a defect.

FORBES: Don't Freak Out If You Hear Your Trust Is Defective