Per IRS Publication 929 Tax Rules for Children and Dependents, on page 9:
The two rules that follow may affect the tax on the unearned income of certain children.
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.