The type of humor the mother uses when speaking to the boy is irony.
Expressing a meaning that is contrary to the words' literal or expected meaning is known as irony. In this instance, the mother gives the boy seemingly simple instructions to "collect our Nigerian goat," but the irony is that the goat is called "Nigerian" during a discussion that contains disparaging remarks about Biafrans.
The goat's designation as "Nigerian" stands in ironic contrast to the earlier derogatory remarks made about Biafrans.