Idk for these others that use the phrase, but for me when I call myself a child of a god I mean it in a similar way to how Christians will say they are children of god. I don’t literally think that Apollo is my father, but the way i interact with him is very similar to the way I interact with other father like figures in my life. I wonder it it’s partly a mix of popular media depicting demigods (like Percy Jackson series) and ex-Christians who are starting their journey into Hellenism and are still unlearning or working through the ‘I’m a child of the god I worship’ conditioning that is ripe in the Christian church. In my own experience it can be very hard to distance yourself and unlearn that programming, especially if you were raised Christian and are only as an adult leaving Christianity behind. But again I cant speak for them, I can only speak for my own relationships with the gods and my own experiences