Can other people see my son's/daughter's information?

No. Only parents and guardians are issued access to the Parent Portal. As long as you protect your username and password, others will not be able to see your child's information.  
If you want to grant access to others (grandparent, aunt/uncle) to view your child's grades via Parent Portal, you will need to provide them with the student's access ID and password that was provided to you by your child's school. It will be the responsibility of each parent/guardian to distribute (or not distribute) this access as they see fit.  
Another option within the Parent Portal is to set email notifications for multiple email addresses. These email notifications send recipients requested information such as current grades, detailed reports of assignments and scores as well as attendance. To set up email notifications, see the "Email Notification" section of the Parent Portal User Guide.
Note:  When adding multiple email addresses in the Email Notification section of the Parent Portal, make sure they are separated by a comma and no trailing space.