Dynamic content by using keys on a key

Keys are static, they are just store translations.
If what you need a key that has "xx is child of yy", where xx and yy are controlled by your game (e.g. NPC names, tools, etc), then you should use parameters or callbacks for that.
( inter-illusion.com/forum/i2-localization...-multiple-terms#2146 )

Its pretty easy to setup and gives you total freedom in what operations you do to make the replacement of xx and yy.

Just make the KEY_C translate to "{[NPC_A]} is child of {[NPC_B]}" and then setup your parameters script to return the correct values for NPC_A and NPC_B. Once you setup the parameters, the plugin will automatically replace those string with the correct value. Remember that you can have NPC_A & NPC_B as global or local parameters.

Also, after thinking a bit more, If you really need to have Term setup that way. You can do it easily in the Spreadsheet: You can make a formulae in the spreadsheet that replaces the text with the content of another cell.

Then, everytime you import into Unity, the KEY_C value will have the string formed from both KEY_A and KEY_B.

Hope that helps,
Frank