This is not correct. Both words are used for people.
Difference I think is that “bolna” is translated as “to say”, while “kehna” is translated as something like “to explain” or “to tell”.
As in:
" wo bola, ye kya he?" (he said, what is this?)
“mein ne uss se kaha ye baat nahin he” (I told him this is not it)
May be I am wrong in translation of the words. May be they can be used interchangeably. But I am certain that the difference is not about animals and humans.