If you have set restrictions on your child's Apple device, the app may show an error when you open it that refers to 'localhost'.
To fix this problem, please add an exception for "localhost" in the website restrictions on their phone or tablet.
Go to Settings > Screen Time.
Tap Content & Privacy Restrictions and enter your Screen Time passcode.
Tap Content Restrictions, then tap Web Content.
Tap Allowed Websites Only.
Tap Add Website.
Enter a Title, e.g. 'TTRS' and a URL, which must be 'localhost'.
Tap <Web Content to save your change.
You should now be able to open the app!