Use the locator that is already on the phone before you pay for anything: Google Family Link gives a parent free, continuous location on a child's Android phone, and Find My does the same for an iPhone in your Family Sharing group. Both are free, both keep working through OS updates, and neither requires hiding an app from your kid.
What free actually covers. In Family Link you add the child's Google account as a supervised account, then get a live pin, 24-hour location history, place alerts for home and school, and a battery readout. On iPhone the equivalent is Settings > [child's name] > Find My > Share My Location, plus Screen Time for the usage side. Where the free tools stop. Family Link answers "where", not "what". It does not show SMS contents, call logs, installed apps, or browser history, and it is trivially visible to a teenager who knows the settings menu. That transparency is a feature, not a flaw, but it means the free tier leaves a real gap once a kid is old enough to use messaging apps heavily.
What the dashboard adds. Ultimate Phone Spy runs on Android 4.0+, Windows and Mac — it is not an iPhone product, and I would rather say that plainly than sell you a subscription that cannot work. On an Android child phone it adds SMS and call logs, web history, installed apps and location history in one place; details on the location tracker page and parental control features. Pricing is $69.90/month, $40/month billed quarterly, or $16/month billed annually — see pricing. Installation takes a few minutes with the phone in your hand, and it is designed to sit on a device you own and administer, with your child aware of it.
Suggested setup. Free tools for the iPhone, dashboard for the Android, and a written family agreement about what is checked and when. If the phone is a partner's or another adult's, stop here: installing monitoring software on an adult's device without consent is a criminal offence in most jurisdictions, not a grey area.
We compared the free and paid options in more depth in this thread.