Because a child's unearned income (interest, dividends and investments) can be taxed at the parent's rate (to keep parents from dodging taxes by putting investments in their children's names), a child's standard deduction is calculated differently. If their only income is earned from working, then their standard deduction is $12,900 or equal to their earned income, whichever is less. If they also have unearned income, it gets more complicated.
https://www.irs.gov/taxtopics/tc553
