In most cases, there is no need to report interest income that is reported in a child's SSN on 1099-INT. If the amount is under $1,100 for unearned income and that was their only source of income, it doesn't need to be reported on any tax return.
Here is a TurboTax help article with more information on Reporting Interest for a Child if the unearned income from interest, dividends and capital gains is over $1,100.