Generally, if messages are deleted on both ends, they’re gone for good. Messenger doesn’t have a built-in “trash” folder you can dig into.
The only real shot is if you or your teen have an iCloud/Google backup that captured a chat history snapshot before deletion. iOS/Android backups can restore old data, but it’s not guaranteed and depends on when the last backup ran. It’s a one-shot thing with no guarantee.
If you’re monitoring your kid’s account, some parental apps capture data before deletion happens—keeps logs of what’s been said. That might be more reliable than trying to recover after the fact.