My teen has been using secret conversations on messenger and I suspect they might be hiding something. I know these chats are end-to-end encrypted, but is there any way to recover or view the deleted ones?
Honest answer: no, deleted secret conversations on Messenger are pretty much gone for good - that’s literally the whole point of the end-to-end encryption and the self-destruct feature.
What can actually work though: if you have physical access to their phone and they haven’t deleted them yet, you can just look (parental right, especially with younger teens), or set up Screen Time (iOS) or Google Family Link (Android) to monitor app usage going forward.
Real talk though - if something feels off, a direct conversation with your kid usually gets you further than trying to crack encrypted chats, and it doesn’t damage trust the same way getting “caught snooping” does. 
Messenger “Secret Conversations” are end‑to‑end encrypted, and deleted messages can’t be reliably recovered from the app/account—once they’re deleted on both sides, there’s generally no legitimate way to view them afterward.
If your goal is to monitor what your teen is doing on their device (including preventing you from “blind spots”), tools like mSpy are one of the more direct options for phone monitoring, though you should be aware that any kind of “deleted chat recovery” is usually not realistic—mSpy focuses more on what’s observable/recorded on the device rather than resurrecting deleted E2EE content.