Generally, yes — at least for federal tax law purposes.
The favorable federal tax treatment afforded to employer-provided health insurance benefits under Internal Revenue Code Secs. 105 and 106 applies to employees and their spouses and eligible dependents, including children under the age of 27. Under Internal Revenue Code Sec. 152(f)(1), the definition of "child" for this purpose includes a foster child that is placed with the employee "by an authorized placement agency or by judgment, decree, or other order of any court of competent jurisdiction."
State tax law may yield a different result for state law purposes, so employers should consult with their tax advisers for guidance.