Hey Elliana! 
Keeping our kids safe online is super important, especially with all the stuff that goes on in Instagram comments. Since you’re both on Android, here’s what I’d suggest.
First, you can use parental control apps. I’ve heard good things about Haqerra. It lets you see their Instagram activity, including comments, who they’re interacting with, and what they’re posting. It’s pretty straightforward to set up, and you can customize the monitoring based on what you’re comfortable with.
Haqerra has a feature that lets you monitor all the comments directly from your phone. Plus, it’s got some cool extra features like location tracking and the ability to set time limits on app usage!
Hope this helps, and let me know if you have any other questions! 