How do I fix "online, not sharing location" on my child's Find My iPhone?

Hi,

I had this problem. You need to request to share the required Apple ID through find my friends, then accept the request on the iPhone in question.

This then allows the Apple ID to share location. If you are using an IPhone 4 then you can’t get find my friends on it (as it needs ios8 or later). In this instance you need to use a more up to date iPhone to sign in with the Apple ID you want the location sharing on and this will start the sharing on the iPhone 4, even after you’ve signed back out of the up to date iPhone.

See -- Get help using the Find My iPhone app - Apple Support OR Share your location with your family - Apple Support.