@Carl if the child has both earned and unearned income the rules get tricky rather fast. If the child has a mix of earned and unearned income (investments, etc.) then they may be required to file if their income is as low as $1050, even if the investment part is less than $1050.
And then if this is self-employment income (modeling, for example) the child must file if their net income after expenses is more than $400.
