how to unlock child protection lock?

You meant you’re currently being locked out by child mode on your Redmi Note 4? What version of MIUI are you running? Could you go to the device’s Settings > About phone > MIUI version field to check on this. As far as I know child mode has been removed since last year’s update.

Anyway, have you try unlocking the child mode using your privacy protection password. The old version of MIUI used the Privacy protection password for child mode, app lock and most things.

If you have forgotten your Privacy protection password, you can also check out the old tutorial about resetting and recovering Privacy protection password here to find out which one still works for you.
http://webcazine.com/19186/miui-what-to-do-if-youve-forgotten-your-privacy-protection-password/