I'm just wondering exactly why it was called this? Surely it could of been called "adjust fee" or something simpler?
No, it could not have.
Child Pays For Parent is called such because you are creating a new transaction which spends from the transaction that you want to change the fee of, hence it is a child transaction of the parent unconfirmed transaction. This child transaction contains a high enough transaction fee to pay for the parent. Hence Child Pays For Parent.
The other "adjust fee" method is called Replace By Fee. With Replace By Fee, you are creating a conflicting transaction that pays a higher transaction fee and replaces the original lower fee one.
So because there are multiple methods to increasing the transaction fee, it can't just be called "adjust fee".
