A new tax law signed by President Bush further tightens the screws on wealthy parents who make gifts to a child in order to take advantage of a lower tax rate on children's investment income. The law marks the second time in just over a year that Washington has extended the reach of the so-called kiddie tax, which subjects a child's income to a parents' higher tax rate. What prompted Congress to further expand the kiddie tax were reports that some wealthy parents were planning to take advantage of a tax-law change designed for low-income people that will be effective next year.