Sons were also born to Shem, whose older brother was Japheth; Shem was the ancestor of all the sons of Eber.
Genesis 10:21 (NIV)
Jewish commentaries say the phrase "were also born" is a narrative device that's meant to come after a pause. The readers should wonder if Shem will have children. The writer delays giving the answer.
"Blessed" Noah had no more children, so how will God fulfill his Promise to bless the world? Would the blessing on Shem be the same or something different? And now we have the answer. Shem will also have children.