Please navigate to Android Kiosk Lockdown > Peripheral Settings > Display. You will have the below option under Brightness:
Users can change brightness: Allow users to adjust brightness on their devices.
Keep current brightness: Forcefully locks the device’s brightness to the current brightness set on the device. Please note that users can change the brightness level if the option ‘Enable status bar’ s checked.
Set brightness: Enables admin to adjust the brightness of devices from the console. Enabling this option will clear any PIN, unlock pattern or password saved on the device.
Please make the choice based on your requirements, associate targets and save the policy.
Kindly use the link for more insights into the peripheral settings