Your child has their own Google account, private email address and password.  

To access the Google classroom you should either go to google.com and login to the google account on a desktop computer or download the google applications on a phone or tablet and login to your child's account.

Please do not change their password.  You must always ensure that you are logging into their account to access the google classroom, email, drive and documents.  You will not have access if you are trying to access their account and data from your own personal email account.