Honestly, there’s no truly “invisible” app on a stock iPhone without jailbreaking or using Apple’s enterprise tools—and jailbreaking opens you up to all sorts of instability and security holes. The easiest, 100% legit way is Family Sharing’s Location Sharing (Find My): your kid or partner taps “Share My Location,” and you see them anytime—no extra installs, rock-solid privacy and reliability.
If you really need stealth for a device you own (say, a kid’s school-issued iPad), you can supervise it via Apple Configurator (wipes the device) and push down an MDM profile that silently reports location. Otherwise, you’ll have to jailbreak or look into Android, but Android-side loaders get auto-flagged by Play Protect, drain battery fast, and are easy for anyone to spot in settings.