There is no app that reads Snapchat messages on an iPhone, and any listing that claims it is misrepresenting how iOS works. Snapchat's content is sandboxed inside the app, encrypted in transit and designed to disappear after viewing, so no iPhone tool - ours included - can pull another person's snaps, chats or friend list.
Start with what Apple and Snapchat give you free, because for most parents it is enough. Snapchat Family Center (Settings > Family Center, free) shows who your teen is friends with and who they have communicated with over the last seven days, plus a way to report accounts you are worried about; it deliberately does not show message text. Screen Time covers the behaviour side: App Limits on Snapchat, Downtime, and Communication Limits that restrict who can be contacted during downtime, all under Family Sharing from your own iPhone. Neither gives content, and that is a policy decision by Apple, not a bug.
About our app, plainly: Ultimate Phone Spy runs on Android 4.0+, Windows and Mac and does not install or function on iPhone at all - iOS does not permit device-level monitoring, so we would rather lose the sale than sell an iPhone licence that cannot work (system requirements). Where the phone in question is Android, or the concern extends to a family PC, the dashboard covers SMS and call logs, GPS location with history, browser history and the full list of installed apps with install dates (installed apps, location tracker).
Judging every other vendor's iPhone Snapchat claim: ask for a demonstration on your own iPhone before paying, and listen for "iCloud backup extraction" - that route only works with the target's Apple ID credentials and 2FA, which you cannot lawfully obtain from another adult. "No jailbreak needed" and "live Snapchat messages" in the same sales page is the tell. If the teen is on iPhone, Family Center plus Screen Time plus an honest conversation is the real toolkit.