What's the deal with all the Dragon kids?
This is an alternate viewpoint from the conventionally held theory that Seath fathered Priscilla and (somehow) Gwyndolin. I am assuming Priscilla is Velka's daughter, fathered by Gwyn, which if you disagree with, that's completely understandable.
Thematically, I believe this is appropriate as Seath is something of an alchemist seeking to artificially manufacture life, and the prospect of the natural generation of life seems like it doesn't really fit with him.
It presumes Gwyndolin is Gwyn's son, and it takes into consideration the fact that the first born son of Gwyn has an extreme affinity for Dragons
This theory depends mostly upon a read of environmental details, as should be evident from the image. Yes, I'm aware of the spelling mistake, no I will not be changing it, as I think it adds to the MS paint aesthetic.
This theory would also explain why humans would be capable of partial (and potential total) transformation into Dragons
Theme of Degeneration
In mythology, it is a recurring theme to think of the past generations as greater than the current ones. A great example comes from the Iliad, where characters speak about Heracles with great respect, as someone more heroic, more powerful, and more godlike than those around in their modern time, and in the recounting of the Iliad, characters such as Achilles would be treated the same by the Greek listeners. A degeneration of man has occurred, we are now shorter, weaker, less godlike.
This theory meshes well with those notions, and explains why someone like Gwyn could go Hollow, despite being a god, as the gods are not substantially different from the humans, just bigger, stronger, and more powerful.
Pygmy Admixture?
One potential reason why the godlike beings of the older age could be shrinking in size is the proliferation of the Pygmy race
Could the Dark Soul be stripping away the more Dragon-like aspects of the inhabitants of Lordran? Does becoming a Dragon divest someone of their portion of the Dark Soul.
Conclusion
I'm throwing darts at a board, what are your thoughts?