1099b - Custodial Account

Neither. The child needs to file it. A custodial account is under the child's social security number. Since he is your child and dependent, there  is a provision for entering it on your (not the aunt's) return, using form 8814. But, this can only be done if his only income is from interest and dividends, Alaska PFD or capital gains distributions shown on a 1099-DIV.

Since he has a 1099-B, for the sale of securities, it has to go on his own return. If your dependent child is under age 19 (or under 24 if a full time student), he or she must file a tax return for 2015 if he had any of the following:

1.         Total income (wages, salaries, taxable scholarship etc.) of more than $6,300 (2015).

2.         Unearned income (interest, dividends, capital gains) of more than $1050 (2015).

3.         Unearned income over $350 and gross income of more than $1050

4.         Household employee income (e.g. baby sitting, lawn mowing) over $1900 ($6300 if under age 18)

5.         Other self employment income  over $400

Even if he had less, he is allowed to file if he needs to get back income tax withholding. He cannot get back social security or Medicare tax withholding.

 He doesn’t get his own $4000 exemption (deduction), when he files. In TurboTax, he indicates that somebody else can claim him as a dependent, at the personal information section.