After connecting, the child app asks you to make up a four-digit PIN. You know it, the child doesn't: without it the permissions and protection on this phone can't be changed. If you forget it, set a new one from your phone: “Devices” → “Forgot the PIN on your child's device?” (a linked e-mail is needed).
The settings PIN
The PIN and Android permissions. Every step has a “How to do it” button. For the child