Or HaChaim on Numbers 3:1/אור החיים על במדבר ג׳:א׳

1 תולדות אהרן ומשה, descendants of Aaron and Moses, etc. Curiously, the Torah enumerates only Aaron's descendants. This is to tell us that they were considered as if they had been Moses' descendants seeing he had prayed on their behalf as we know from Deuteronomy 9,20 where G'd is reported as having been angry at Aaron, ready to exterminate him. This would have resulted in his remaining two sons dying. Our sages in Vayikra Rabbah 7,1 say that Moses' prayer helped to keep Eleazar and Ittamar alive.

2Our verse continues ביום דבר ה׳, to tell us precisely when Moses prayed on their behalf. There is also a hint here of the reason that his prayer was effective as the Torah's mention of Mount Sinai is a reminder that he was such a loyal representative of G'd that he had been found worthy to receive the Torah on Mount Sinai.