If the minor's total unearned income (such as investment income), including the dividends, is more than $1,150 (for 2022) the minor is required to file a tax return. If the minor is too young to file by himself or herself, the minor's parent is responsible for filing the minor's tax return. The custodian of the UTMA account is not necessarily the minor's parent. Filing the tax return is the responsibility of the parent, not the custodian.
