I think the issue is this. If you have earned income greater than half of your support the IRS assumes that the income was in fact used for your support. In that case you can’t be a dependent. So you are either a dependent and subject to Kiddie tax or you can’t be claimed as a dependent and are not subject to that tax.
There is an IRS worksheet to determine support.
https://apps.irs.gov/app/vita/content/globalmedia/teacher/worksheet_for_determining_support_4012.pdf
