Each account you create, can be reached in the same ways you see in your setup email for your own account. The only difference is that you substitute their username and password for yours.

http://www.yourdomain.com/~username/ to view the account.

http://www.yourdomain.com/cpanel/ to enter Cpanel

Pop3 and SMTP settings are the same as yours, mail.theirdomain.com or the IP address detailed in your setup email.

It's important to remember that all accounts, while they vary in size, are essentially the same. The same methods of access work on all accounts, just with different usernames and passwords. The difference between you and your clients is that you have access to WHM and they don't.