In iOS 13 and above, Apple has made changes to disable background location tracking option inside the app. Now you have to manually turn On the feature by going to:-
Settings > Privacy > Location Services, tap 'FamilyOrbit', and select Always
If this setting is not turned On, the background location will not be fetched by the App.
Additionally, Apple iOS periodically prompt the active user to approve location sharing, if the child taps on the option to turn off location sharing, the app stops sending the location.
To fix it, on the child's iPhone open Settings and scroll down to FamilyOrbit - tap on it. Change option of Location Services to On, and rest all options to On as well. Run the app afterward once to sync to the new location.
To prevent your child from turning off location or removing the app, you can make use of Apple's Restrictions features, read more about it here:-