The Torah enumerates Reuven and Shimon's children by name, but not their grandchildren. However when it comes to Levi's descendants the Torah lists his children, grandchildren and even his great grandchildren.
Ramban explains their distinction. He says these progeny of Levi were especially great, so great that they were worthy of being enumerated by name, as were all the original 70 souls (which included Reuven and Shimon's children) who came down to Mitzrayim. This is true even though Levi's grandchildren were in fact born only after they descended to Mitzrayim.
But Ramban does not explain why Levi's 3rd and 4th generations were as great as his brothers' children.
Sforno explains that since Levi lived so much longer than his brothers (137) he helped raise his grandchildren and great grandchildren, and it was this influence that enabled them to become so great - reaching a greatness that obliged the inclusion of their names in the Torah with those of the previous generation!