Question: Okay, so is it possible to have true Titanic Scions? I remember reading in the Grecian deluge myth that Prometheus had a demigod son.
No, it is not. Scions are specifically the children of gods; they are free of the taint of the Titans and made of equal parts divinity and humanity. Titans can corrupt humans to be their servants or have sex with humans to create Titanspawn offspring, but they can't make Scions. That ability lies with gods alone.
The person you are thinking of is Deucalion, so let's talk about him a little bit! There are actually a couple of Greek deluge myths, but Deucalion and his wife Pyrhha are the stars of the most famous one, in which Zeus floods the earth to destroy the people of the Bronze Age and only Prometheus' timely warning to his son allows the two of them to create an appropriate vessel and escape death. Deucalion and Pyrhha then repopulate the earth and foud a line of Grecian kings.
Greek heroes aren't always clearly labeled, and Deucalion is no exception. Apollodorus refers to him as "god-like", while Hyginus implies him to be part of the "human race"; Nonnus muddies the water by saying that "all mortal men perished" but Deucalion remained to repopulate with his wife. All different writers agree that Prometheus was Deucalion's father, while his mother is sometimes given as Pronoia (an Oceanid daughter of Oceanus and Tethys) but often goes mentioned. Deucalion does a lot of things that are Scion-esque, including receiving aid and favor from the gods to try to save himself from the flood, and he's considered to be dead but the manner of that death is never discussed, implying that he might have died of old age, a distinctly mortal condition. Pyrhha is clearly half-human, half-Titan, as a daughter of Epimetheus and Pandora, although depending on your take on the myth of Pandora's creation you might or might not consider her human.
All this certainly sounds like a Scion, but while we're totally down with running Deucalion as a Scion if you want to, we're not down with running him as the Scion of a Titan. That simply doesn't exist in Scion's framework - and shouldn't, in our opinion, because the creation of Scions is an important and unique feature of humanity that doesn't make sense for Titans, who are by definition no longer capable of interacting with humanity in that kind of close and positive way. This is a lot like that time we talked about Demeter trying to burn the mortality out of a kid with fire; Greek mythology (and every other mythology) occasionally does things that just don't fit into the game's world anywhere easily, because religions and gods are weird and you'll never be able to create a system that covers every ridiculous thing they've ever done, so if you want to reconcile it as a Storyteller, you have to be creative.
The first and most interesting option we'd explore is that Prometheus maybe wasn't a Titan at this point in time. While Pandora's existence tells us that this is after all his shenanigans with the stealing of sacrifices and dissemination of fire and earning Zeus' eternal hatred, Deucalion himself might still have been born before the battle lines between the two were drawn. Zeus himself helps Deucalion, which suggests that there are no hard feelings there. Prometheus is of course definitely of a Titanic generation, but he's also the same generation as Zeus (they're first cousins), so it's theoretically possible that he wasn't a Titan from the beginning and only defected later. Certainly he doesn't appear to have fought in the Titanomachy, and he's generally a more benevolent and unthreatening figure where humanity is concerned. If you go with that option, Deucalion really is just a normal Scion of Prometheus - possibly the only one that ever existed.
Another possibility is that Prometheus is Deucalion's father figuratively instead of literally, and that he's someone else's Scion (perhaps Zeus'?) that Prometheus acts as Guide or mentor for. We're not as big fans of this approach since Greek myth is very clear about repeating that Deucalion is Prometheus' son, but for those who want him to be Titanic from the very beginning but also want Deucalion to be a true Scion, this may be a useful compromise.
Or, you can also decide that Deucalion wasn't really a Scion at all, and proceed accordingly. He's very closely associated with humanity and rebuilding it after the flood, so maybe he was a Legendary mortal, created by Prometheus through the use of Artistry or other creative powers (which would explain why a mother is mentioned so seldom). Maybe he was a normal human that Prometheus decided to adopt, possibly because he married Pyrhha and therefore Prometheus felt some responsibility for her since he made her mother Pandora. Maybe he was actually a lesser immortal or Titanspawn of some kind or even a god, living in the World before the time that the gods left it for good; if his mother is an Oceanid, he could be one of the tritones, or any other kind of supernatural figure associated with water, which certainly couldn't hurt him in his quest to survive the great flood. The mortals that Deucalion engenders to repopulate the world aren't actually children of his but rather created from rock and earth, so he wouldn't need to be human in order to "create" human children in this way.
Honestly, this is one of those places where game rules don't match up and it's a pain in the ass. Use your best judgment, and best of luck to you.