Err... This has been implemented a few months ago already... Just make sure to un-check the "Remember login credentials" check box on the login screen, and do login (at least once) with the avatar on the grid you wish the see the credentials forgotten. After a successful login, the credentials file (user_settings/saved_grids_login.xml) gets your avatar entry removed for that grid, and of course the saved settings will not have your avatar name and password hash stored either.
If you want to secure such things, you'd better be using a separate user account for each household member (each protected by a password and without admin privilege)... Then no one can use another person's file without their permission (the viewer settings and configuration files are kept in a directory that pertains to the user account, so no risk to see them shared by mistake).