Besides Hilda the Mongrel, an Elezen/Hyur mix, we haven't seen any. It's possible, but rare enough that you don't just see mixes running around. A while back, I saw someone mention over on the Japanese side of the Lore forums (which doesn't really talk about lore at all) mention that the child would always be the race of the mother, but they didn't have a source so it was probably just speculation.