To create a child material either copy an existing one and use this method or use the in engine method - this also allows you to change parameters that may not be present in the existing one

Via UAssetGUI

• Download UassetGUI by atenfyr
• Set the version to the version of UE4 your game uses
• Open the target material and search the exports for parameter values
• These can be changed in any way you wish
• File < Save as and recreate the file structure like normal
• Delete the .bak files (and the .uasset because you only changed values) and package