I already filed my 1040. I need to file a second 1040 for my teenaged daughter. How can I file this 1040 on her behalf through my account?

Using the TurboTax online editions you can only complete one tax return per account and User ID.  You would need to create a new account and a new User ID for another tax return.

Go to this TurboTax website and click on Sign up in the upper right of the page to start a new account - https://turbotax.intuit.com/personal-taxes/online/file-your-own-taxes/

You cannot.  You will need to create a second account for your daughter.  You can only file one return per account.