The Lore of Inter-breeding

[Enkidoh]

2014 Las Vegas Fanfest Camate-Fernehalwes Interview:
Q: “Are there any folklore stories involving cross-racial relationships? Especially between Hyur and Elezen? And then, what do Roegadyn-Lalafell children look like? What is the fatality rate for such a union?”

Fernehalwes: “... As for the Roegadyn-Lalafell one, I mean, you know… First of all, logistically, how are you going to… I mean, I’m sure there’s fan art out there on the net. You can take a jaguar and a lion in a lab and do that. But the thing is, it doesn’t happen in nature, because some things shouldn’t happen in nature. But it could. I’m just saying, some things shouldn’t happen but that’s not saying it can’t. It can.”

the issue is more of a wailord on skitty sort of deal than some biological incompatibility like the other person implies. or to use non-pokemon meme example, chihuahua on great dane; they're unlikely to physically mate due to size difference.

Even before then, Ferne explained about the difficulty in mixed race unions producing offspring and that if it does happen, its very rare - on this very forum in fact:

Can different races crossbreed?

Answer: Yes, but it is rare...and often difficult due to cultural differences, etc. which is why you don't see it often. The lore team has informed me that there will be future quests/events that touch on this and other issues, such as love and marriage.

Of course that was done way back in 2011 when ARR was early in development and 1.0 was still active and operating, so like much of EW's plot points there might have been some changes in the lore behind the scenes in regard to this that we don't know about, but my gil is still on that still being canon.

So, having had two citations straight from the head of localization (and one of the main lore writers), it's pretty conclusive that mixed racial children in Hydaelyn/the Source (and by extension, the Shards as well) is actually very rare - anything else is just headcanon at best and players making things from whole cloth at worst.