Every time you create an account, you are creating a full-featured hosting account very similiar to your own main reseller account. The only difference is, you have access to WHM and your client does not.
Your clients can access their hosting accounts through the same methods that are in your setup email, or if your domain has propagated completely they can reach their accounts by using your domain.
http://www.yourdomain.com/cpanel/ - Enter your customers username and password will open your customers control panel.
http://www.yourdomain.com/~clientusername/ - This will allow your customer to see their account prior to propagation of their domain.
NOTE: the tilde (~) is not a typo. This is required for your customer to reach their account.
The mail and FTP settings should be the same IP that you found in your setup email, or after propagation of their domain, your customers may use ftp.theirdomain.com and mail.theirdomain.com