Moses had sent his wife, Zipporah, back to her father Jethro. in the land of Midian, after the incident involving circumcision at the inn, Exodus 4:24-26.
When Jethro comes to see Moses, Exodus 18:1-27, he brings Zipporah with him and her sons, by Moses.
At the time the genealogy in Exodus 6 is written, therefore, Moses has no wife or sons with him. Whether he disclosed their existence to his Hebrew brethren ; whether they are not included because they were not, presently, among the general congregation ; whether they are discounted because there is some doubt they will ever return, is not stated.
They are not included, I would suggest, because at that time, their absence made them irrelevant to the expected progress of Israel and the future of the generations.
Since there is no further mention of them in scripture, we can only speculate as to what happened to them. There is no further evidence, in scripture, that I am aware of, upon which to form a valid conjecture.