Most often the child's phone stopped KidPair itself (memory cleaning, battery saving). Android then turns off the “KidPair: breaks and schedules” service or marks it as broken. On the child's phone open “Accessibility” → “KidPair: breaks and schedules”, turn it off and straight back on. Then check the maker's special settings (autostart, battery, pinning in recents). On Xiaomi the “Ultra battery saver” mode does the same — you get a separate notification about it; once it is off, turn KidPair on again in Accessibility.
Right after an app update Android reconnects the service for a few seconds — that isn't a fault, and KidPair doesn't count it as protection being off.