Form 8615 is used in the "child's" tax return to calculate the kiddie tax. Are you subject to kiddie tax, or are you preparing a tax return for someone who is subject to kiddie tax? (You could be as old as 23 and still be a "child" subject to kiddie tax if you were a full-time student.)
In TurboTax Online you don't see the actual forms until you print the tax return or create a PDF. If you need to enter the kiddie tax information, type "Form 8615" (without the quotes) in the Search box, then click the link that says "Jump to Form 8615." When you enter the necessary information, TurboTax will fill out the form.
