Bethelehem’s population was quite small in those days. There weren’t likely a ton of children who were killed. Nevertheless, they were slain.
Herod was a ruthless tyrant. Josephus and the Talmud were not exhaustive accounts of his life, nor did they have any interest in preserving anything that bolstered the Christian’s argument.
I would not discount something because it isn’t mentioned in the Talmud or Josephus. Many things are not, yet are historically true.
A wholesale massacre of all the Jewish children in a specific town is something that the Jews would remember, not just Christians.