It's somewhat optional. Assuming the account is under her Social security number, it's always OK to file the income on her return; even before she was 18.
If
your dependent child is under age 19 (or under 24 if a full time student), and her only
income is from interest and dividends, Alaska PFD or capital gains
distributions shown on a 1099-DIV, there is a provision for entering it on
your return, using form 8814. Enter at Less common income / Child's
income.
But, it is usually best for her to file a separate return, as qualified
dividends and capital gain distributions could be taxed at a higher rate on the
parent's return.
