Remove/deny public user read permissions on the parent asset, and set to cascade the permission changes. Depending on what permissions the child assets have - you might need to try to use the manually cascade option.
Then grant the appropriate permission (eg read) to the user/user group - on the same asset, and set cascade.
The default Matrix login would would be used if you havent created a custom one.
If you want to create your own custom login page you would created a new design and use the Login Form Design Area, and could also you the Password Reset one as well if required.