Lock it in

Everything so far can be undone by a child who deletes the app. This is the step that stops that, and it's the one most people skip.

iPhone

1. Settings → Screen Time → Use Screen Time Passcode. Use a code your child doesn't know. You can reuse your Brickify PIN digits.
2. Turn on Content & Privacy Restrictions, then set Deleting Apps to Don't Allow.

The full walkthrough: iOS setup guide.

Android

Use Device Owner mode. It makes Brickify impossible to switch off or uninstall. It starts with a factory reset, so have a second device handy for the instructions. Follow the Device Owner setup guide.

Both: add an alert email

Brickify emails you if something needs attention: the phone goes silent, its protection gets removed, or a new app appears despite blocking.