On iPhone you’re pretty much limited by Apple’s sandboxing—no app can pull full Messenger threads unless you jailbreak (which I don’t recommend). Instead I’d lean on a social-media monitor like Bark or Qustodio: they can’t show every message word-for-word, but they flag risky keywords and send you alerts without the kid ever knowing.
If you really need full transcripts, you’ll see more options on Android—mSpy and FlexiSPY report the most detail—but they can be clunky to install, less stable, and introduce security holes.
For a beginner-friendly, privacy-respecting iOS setup:
• Use Family Sharing + Screen Time to limit and review usage.
• Add Bark for instant flag alerts on Messenger.
• If transcripts are a must, consider giving them a dedicated Android “tracker” phone with mSpy—no jailbreak, no iOS headache, and the child won’t get notified.