The IRS interest rate on underpayments is set quarterly. For the third quarter of 2020 (July 1 through September 30), the rate for an individual is 3%. Interest compounds daily. It’s wise to request an installment payment agreement to get outstanding taxes under control. Interest will continue to accrue on the balance, but eventually, the debt will be gone.
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.