[Phantom Stranger]

Obviously that depends on the Rider.
In the case of say, Shin, he actually did have a baby and it was half mutant Grasshopper. I think we can assume the same or at least similar would hold true for any Rider with mutant like physical changes, like Black, and possibly Amazon.

For another example you have Kiva's son, Masao Kurenai. We can probably assume he's at least a quarter Fangire, though that's more to do with naturally being infused with powers rather than undergoing a change from an outside force.