They can file their own returns for it now. Is their ssn on the 1099Int? See these two articles
How to pay the kiddie tax
https://ttlc.intuit.com/community/my-tax-return/help/how-do-i-report-and-pay-the-kiddie-tax/00/26055
Report child investments income
https://ttlc.intuit.com/community/investments/help/how-do-i-report-my-child-s-investment-income/00/2...
