your child is never too young nor too old to file income taxes if they have earned income or they have income from savings or investments. Their age doesn't matter, the amount that they earn is what matters. Capital gain is unearned income. Unless her unearned income exceeds $1,050, she , as a her parent?s dependent, does not need to file her return. The Kiddie Tax is applied to the amount of your child's unearned income that exceeds $2,100. The Kiddie Tax rate is your marginal tax rate which could be as high as 39.6%. kiddie tax is a tax rule that is levied on unearned income earned by children under the age of 19 and college students under the age of 24. ... All of the child's unearned income in excess of $2,100 is taxed at the parent's tax rate.