You will get concerned registration code/license-key (to install SecureTeen Child App) inside of downloaded SecureTeen Parent-app, on your personal/parental device. For this, you are supposed to check given 'View Instructions' against available child-profile(s) within Parent app.
Whereas; if you need some old registration-code then please tap on given 'Edit' icon (pencil-icon) against already available devices/child-profiles on dashboard and go to Reinstall Child App option on next screen.
While to get new registration-code, you will need to add a new device/child-profile on dashboard first. To create new child-profile you will find a Plus "+" signed tab/button on dashboard of Parental app. Once done, please tap on available 'View Instructions' tab under this newly added device to get requires registration-code/license-key.