When the text speaks of "the sons of Kohath" (vss. 2, 4, and 15), this is not meaning specifically Kohath's physical children only, although that meaning's not excluded. In Numbers 4, "the sons of Kohath" means those who are his descendants. The Levites were set aside to minister the religious aspects of Israelite life, and the Kohathites were set aside for this sub-specialty in the priestly service.
The phrase "sons of" doesn't always mean physical descendancy in OT usage. Amos 7.14 uses the phrase, but in that sense it means not physical lineage, but one who is a student of the prophets, or trained to be a prophet. In other words, Amos was a herdsman, and like Melchizedek or Christ, he is one not expected to be a prophet. The sons of Levi, the ones who would be expected to be prophets, are mediately called through their physical birth into the Levitical line, but Amos is given an immediate call, while he was herding the flock.