Thanks, however I seem to be unable to do this via the GUI. Even when selecting outputs manually it only shows those that have confirmations, I cannot choose an output that is unconfirmed.
So it seems CPFP is impossible via the GUI.
Yes, it is not available in the GUI, only the RPC commands.
We should probably look into implementing this feature for the GUI then.
There is a shell you can access from the Tools or the Help menu I think, that lets you type RPC commands without having to shut down the GUI and start the daemon (and then do the same again to get back to the GUI), but I think it might be a good idea to use a checkmark for "Use CPFP for this transaction" that is unselected by default.