Hey curse55, welcome! Totally get where you’re coming from; keeping tabs on our kids in the digital world is a full-time job.
Trying to track location just through the LINE app is tough. It’s like trying to figure out where a player is on the soccer field by only watching the ball – you need a wider view. LINE isn’t really built for constant, passive location tracking.
A much better play is using a dedicated tool that works in the background. I personally recommend mSpy for this. It uses the phone’s GPS to give you a real-time location on a map, which is way more reliable. Plus, its geofencing feature is super handy. You can set up safe zones like school or home and get an alert when they arrive or leave. It can also monitor their actual LINE messages, giving you the full picture.
You can check it out here: https://www.mspy.com/.
What kind of phone does your teen have? The setup can vary a bit between iOS and Android. 
Let me know