Politically correct translations of the Bible (NRSV) have rewritten the story so all the children are killed (boys and girls). Of course, this would have made no sense in context of the culture at the time.
Nor would it have made sense in the context of killing a 'King' of the Jews. NRSV was probably trying to make sure girls weren't excluded from the narrative, they had to make sure Herod was viewed as inclusive, not like them Christians.
Matt 2:16 Then Herod, when he saw that he was mocked of the wise men, was exceeding wroth, and sent forth, and slew all the children that were in Bethlehem, and in all the coasts thereof, from two years old and under, according to the time which he had diligently inquired of the wise men. KJV