If a family can't recall their password, they can use the Reset Password link on the Portal login page to generate a password reset email. This email is sent to the email address on the contact's record.
You can also reset a contact's Portal password from the their record in the Family account. In the Portal Information section of the Contact record, click Reset Portal Password. This will generate a temporary password which will be emailed to the contact. It will also be displayed briefly allowing you to copy it and email them directly if you choose.