You have until October 15, 2019 to e-file the 2018 return; after that it has to be printed, signed and mailed. Not sure what you think the extension will "prove" about your dependents. It will not prove anything, except that you requested an extension to file your tax return.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
