Are you sure you don't have anything that's considered "unearned income"? Unearned income is basically any income that wasn't from working. It could be interest from a bank, Social Security benefits, or even unemployment benefits. Here's the definition of unearned income for kiddie tax, from IRS Publication 929, page 15. Although it's not mentioned in Pub. 929, taxable scholarship income is also considered unearned income for Form 8615.
"Unearned income is generally all income other than salaries, wages, and other amounts received as pay for work actually performed. It includes taxable interest, dividends, capital gains (including capital gain distributions), the taxable part of social security and pension payments, certain distributions from trusts, and unemployment compensation. Unearned income includes amounts produced by assets your child obtained with earned income (such as interest on a savings account into which you deposited wages)."
