If you mean “remotely” without installing anything first, the honest answer is: for privacy + security reasons, most legitimate apps can’t magically view another phone’s texts without initial setup/permissions on the target device. The ones that do work usually require you to install an agent once (and you’ll need to weigh that against the “getting caught” concern—monitoring teens stealthily can backfire badly).
That said, mSpy is one of the more straightforward options for phone monitoring/parental control because it centralizes message viewing in a dashboard after proper setup. If you tell me whether you’re on iOS or Android (and whether you can do a one-time install), I can point you to the most realistic path.