Federal income taxes withheld from a retirement plan distribution, which are listed in box 4 of Form 1099-R, are claimed as a credit on your return against the taxes you owe; they’re reported as taxes that you’ve already paid for the year.
The tax on the investment income in excess of $1,700 (may change after 2007) of a child under age 18, based on the parents’ marginal tax rate and computed on Form 8615.