You can use a Parent's Election to Report Child's Interest and Dividend, Form 8814, to report a child's income, but only if the child's investment income includes only 1099-DIV or 1099-INT. Because your child's income includes a 1099-B, you'll need to file a separate return for them if their unearned income is greater than $1,300.
Here is some information you may find helpful: Tax Filing Requirements for Children
