Why do Nephilim need to be the children of Fallen? In the Bible, the Nephilim were brought about under God's orders, and became the greatest heroes and champions of old.
There are a few different interpretations of Nephilim--even within Judeo-Christian mythology (no offense intended to anyone of faith). Popular culture usage, however, generally names the Nephilim as either the product of a Fallen Angel and human mating or a Grigori and human mating. I think the former is the most used, though, given its use through movies, TV and RPGs.
That said, I think Belial's template for Nephilim could also fit the "Sons of God" type of Nephilim just as well with only minor text changes, and, perhaps, changing the Catch.