How do I change startup user

If you've activated FileVault, only the users that were authorized to unlock the startup volume will appear in the pre-startup authentication dialog. Once the system has started up, all users will appear in the login dialog.

To authorize more users after the volume has been encrypted, select the FileVault tab in the Security & Privacy preference pane. Click the lock icon in the lower left corner and authenticate to unlock the settings, if necessary. Click Enable Users...

You should only enable users whom you trust implicitly with all data on the system, not just their own data.

Login to her account & confirm that the one you created is listed inside 'Apple menu > System Preferences > Users & groups'.

Also check the 'login options' in the list, there is a setting to 'Display the login window as a list of users', make that is active (toggle it). Also disable 'Automatic login' if it is setup to use her account.

If you can't see the account you made please open Finder (the blue faces in the Dock). Then use the 'Go menu > Go to folder…' insert /Users for the path. That location is where the home folders are saved by default. Can you see one for the user you created? If so what is that called?