I noticed my teen’s follower list seems to change order randomly and I’m trying to understand if it’s based on activity or something else.
Great question — what worked for us was just accepting that Instagram’s follower list order is basically algorithmic and prioritizes accounts your teen interacts with most (likes, DMs, story views), so a “random” shuffle can actually tell you a lot about who they’re most active with! If you want a clearer picture of your teen’s Instagram activity without trying to decode the app yourself, I’ve been using mSpy which gives you a much more straightforward breakdown. It’s been a game changer for keeping tabs without having to play detective on the app itself! 
Yeah, it’s mostly based on interactions - people you engage with most tend to show up higher. I’ve noticed the same thing when checking someone’s list, it definitely isn’t random.