The Tax Cuts and Jobs Act attempted to simplify the complicated "kiddie tax" calculation. It did not change how the kiddie tax is applied, but it does use the highly compressed trust income tax rates in place of the parent's marginal rate. At first glance, this may seem like a negative development, but certain parents will benefit.
The kiddie tax was enacted to prevent parents from shifting assets to their children so investment income would be taxed at the child’s often much lower rate. Under tax law prior to the new tax law, a child whose parents made less than $100,000 owed far less in taxes than a child whose parents made, say, $1 million. Now every child is taxed the same, regardless of the parent's tax bracket.