Per IRS Publication 929 Tax Rules for Children and Dependents, on page 9:

Part 2. Tax on Unearned Income of Certain Children

The two rules that follow may affect the tax on the unearned income of certain children.

1. If the child's interest and dividend income (including capital gain distributions) total less than $11,000, the child's parent may be able to choose to include that income on the parent's return rather than file a return for the child. (See Parent's Election To Report Child's Interest and Dividends, later.)
2. If the child’s interest, dividends, and other unearned income total more than $2,200, the child’s income is taxed at special tax rates. (See Tax for Certain Children Who Have Unearned Income, later.)

For these rules, the term “child” includes a legally adopted child and a stepchild. These rules apply whether or not the child is a dependent.

These rules don’t apply if neither of the child's parents were living at the end of the year.

Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.