They would have to report the income on their own tax return if required to file. You cannot include a child's income on your return.
2018 Filing requirements for dependents.
Single dependents— (Not age 65 or older or blind)
You must file a return if any of the following apply.
1. Your unearned income was more than $1,050.
2. Your earned income was more than $12,000.
3. Your gross income was more than the larger of—
a. $1,050, or
b. Your earned income (up to $11,650) plus $350.
Or Dependent had net earnings from self-employment of at least $400.
However, if any tax was withheld, then Dependent might want to file anyway to get a refund of the tax withheld.
Also, a person that CAN be a dependent cannot claim him/her self and must check the box that says that another taxpayer can claim them (whether or not that taxpayer actually claims him/her).
Note: your state filing requirements might be different than the federal.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**