Yeah, I looked into this when I first started the whole co-parenting surveillance thing. The short answer is no, not really.
Those “track by number” services you see advertised? Either scams, require the phone carrier to cooperate (which they won’t without a court order), or they’re those sketchy apps that promise the moon and deliver nothing.
If you’ve got an iPhone, your best bet is using Find My with their Apple ID - assuming they’ll share it with you. That’s actually free and works. Otherwise, you’re looking at proper monitoring apps that need to be installed on the device itself.
The “just use their number” thing is mostly clickbait unfortunately. Trust me, I wanted the easy route too when I started this journey.