App to hide text messages?

Two different questions get mixed together here: the app someone uses to hide messages, and how a parent detects one. There is no legitimate app store category called "hide your texts" — the tools that do it are vault apps disguised as calculators, private messaging apps with their own inbox, or notification managers that suppress previews.

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How these apps hide

• Calculator vaults. The icon is a calculator; entering a code opens a hidden photo/message vault. They usually do not register as messaging apps, so they never appear in the Messages app.
• Second-number apps. TextNow, TextPlus and similar give a second number with its own inbox. The messages never reach the carrier SMS database.
• Private messengers. Telegram secret chats, Signal disappearing messages, Snapchat, and Instagram vanish mode all remove content after viewing.
• Notification suppression. Turning off previews, or a "hide notifications" utility, leaves no trace on the lock screen — which is often the first thing a parent notices is missing.

How to detect one on a phone you administer

1. Screen Time → See All Activity → Show Apps lists every app that has been used, including ones deleted later in the week. A calculator showing twenty minutes of daily use is a tell.
2. App Library on iOS: swipe to the last page, check the Hidden folder and search for "vault", "private", "secure", "calculator".
3. Battery usage under Settings → Battery lists apps that ran in the background, even brief ones.
4. Settings → General → VPN & Device Management catches configuration profiles installed to mask activity.
5. On Android, Settings → Apps → Special access → Notification access shows what is intercepting notifications.

The line not to cross

Installing a monitoring tool on another adult's phone, or reading their messages without consent, is unlawful in most countries even if you share a plan. On your minor child's device you have both the legal standing and the responsibility to be transparent about it — a kid who finds a hidden tracker learns to hide better, which defeats the point.