You can send the username as such (with the $USERNAME$ replacement tag used in communications). But you cannot send the password value as such, for security reasons. Instead of the password value itself, when you use the replacement tag $PASSWORD$ in your communications to clients, the system will create a link for the client to (re)set their password.
Note: The replacement tags only work in communications, and not on the website pages.
Also, the system has a special notification template for this purpose (found in My Account > Notification Templates > "Password reminder" template, if you want to edit it), and you can send this notification email to a client from Contacts tab > Contact Name > Profile tab > section "Client Information" > link "Send reset password link to client" under the "Change password" button).
This is how the email will appear to the recipient: