- Android: Use Google Family Link (free). Set up a managed child account and grant all requested permissions (Accessibility/Usage Access); once supervision is active, the child can’t remove it or uninstall without the parent’s approval/password.
- iOS: No third‑party app is truly undeletable; use built‑in Screen Time (free), set a Screen Time passcode, and set Deleting Apps = Don’t Allow plus restrict Account Changes.
- Optional Android alternatives with free tiers: Kaspersky Safe Kids or Qustodio Free, ensuring you enable all required permissions (e.g., Device Admin/Accessibility) for tamper‑resistance.
Share the device model and OS version, and any setup error messages, and I’ll provide exact step-by-step configuration.