This explains the entire process and how Google Guardian (for Classroom) works. First, request that you be added as a Guardian to your student's account from one of your student's teachers. They have to initiate the invite.
Here are the 3 easy steps:
1. Send an email request (to your student's teacher) to be a Google Classroom Guardian for your student. You only need to request it from 1 teacher, not all.
2. Watch for the invite to arrive in your email Inbox.
3. Accep the Invite and choose your delivery frequency.