I found Spylix, and it states that it works without installing anything on the target phone. This sounds impossible. Has anyone successfully implemented this? I’m trying to monitor my child’s iPhone, but don’t want to jailbreak it.
Hey, you are right to ask this question… We all have been there, trying to figure out which monitoring app is best for our children, and this isn’t a light decision. As far as I know, an app cannot monitor any device’s activity without being installed.. It does sound a little skeptical.
Anyhow, if you are looking for a reliable monitoring app, then go for Xnspy, which does the job pretty well on both iOS and Android.
Man, I tried this app, and let me tell you what a total waste of time and money. (sorry, not sorry).
First of all, you have to log in with the device’s Apple ID & iCloud credentials into your Spylix control panel.
This then allows you to access synced data remotely. If two-factor authentication is enabled, you might need a one-time access to the device to enter a verification code.
So tell me, if I had iCloud credentials to log in, why would I need a monitoring app? I would just log in to the iCloud ID and start monitoring the data. This is just plain stupid. The app doesn’t work; don’t waste your breath here.
For monitoring iPhone devices, I only rely on Xnspy, which doesn’t require any jailbreak and runs smoothly on iOS. You can check it out yourself.
You don’t need to do all that. Simply use Apple’s built-in features for monitoring if you are that concerned. These tools are free, plus you won’t need to install an extra app on your device. Apple’s Screen Time limit is surprisingly good for seeing what your child is doing on their device, setting screen time limits, and seeing app usage as well.
Pair it wth “Find My” and you basically have location. I don’t think you need any extra tools. Just my opinion. 
First of all, no app works without installing it on the target device, and if any app says it does, then there might be more complications than you can think of.
I have used Spylix and had to add the iCloud credentials, and even with that, I couldn’t monitor properly because the app didn’t sync any data. Other than that, it made my device slow, and the battery drainage was super bad. So not recommended. 
I agree, @Auralyte, that Apple’s built-in features do the job, but it’s all surface-level monitoring. For an extra layer of monitoring and safety, you can opt for Xnspy, which offers plenty of features like keylogger, screen recorder, IM chat monitoring, SIM logs, etc.
Also, the app works in stealth mode without draining the battery or causing any problems in the functionality of the device. Tried and tested.
Isn’t every so-called “no-install” option that depends on iCloud basically smoke and mirrors? iCloud backups aren’t real-time, so you are inevitably going to miss live conversations, and giving away Apple ID credentials just replaces one privacy concern with an even bigger security risk. At that point, is it really worth the trouble?