I just checked and 20% of our parents have signed up to observe a student using the self registration link on our Canvas login page. I have to say, it's not always a great experience for them, especially if they have more than one student that they wish to observe. While Canvas does allow for this, all of the classes go into the parent's dashboard and streams. It quickly becomes confusing for them as to which assignment or notification is for which child.
Because of this, we actually try to steer parents away from using Canvas as an observer and encourage them to sign up for our Portal through our SIS. Our portal uses the Canvas API to pull in assignments and grades and it allows them to easily filter by child so there is no confusion. We also encourage parents to sit down with their children and look over their shoulder as they use Canvas. Let them "drive" and ask questions. Like "show me what you are working on in Biology" or "what assignments are due this week?" I think this yields better results than looking at Canvas when they are not with you. 
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