Hey, so real talk - Snapchat is tricky because messages delete themselves by design. Most monitoring apps can’t actually capture Snapchat content reliably without rooting/jailbreaking the phone (which breaks warranties and creates security risks).
Your best bet is honestly simpler stuff: check their Snap Map location sharing in settings, use iPhone Screen Time or Android Family Link to see how much time they’re spending on it, and maybe have an agreement where they show you their friend list periodically. Some apps like mSpy or uMobix claim Snapchat monitoring, but they need physical access to install and results are hit-or-miss.
Have you tried just talking to them about online safety and maybe following their public story?