Kaido was born 4 years after Loki,if Harald really his father we should see the mom next chapter. Since Kaido is way smaller than a normal giant despite Harald being huge and he ages normally (he already looked like he went through puberty at 15) the mom was likely human.
Kaido isn't even a giant or half giant, he might have ancient giant blood in him though, the same thing with Big Mom, she is also just a regular human but she might have giant blood in her.
Also I highly doubt Harald is Kaido's father, I don't believe Kaido's parents are going to be important.
Big Mom's parents were just normal people, we know nothing about Whitebeard's parents, not too sure why Kaido's parents have to be important.
Oda does not care about Kaido that much to have him be the son of Harald and brother of Loki and Hajrudin.
Oh and even if he was Harald's son, Kaido would be a giant, and as we know, Kaido is not a giant or even a half giant.