(KJV) Genesis 46:21
21 And the sons of Benjamin were Belah, and Becher, and Ashbel, Gera, and Naaman, Ehi, and Rosh, Muppim, and Huppim, and Ard.
Benjamin is called a lad(Gen 43:8)which seems to rule out any children at this point.
When they leave for Egypt within two years of the famine he has ten sons
As Joseph reveals himself to his brothers(Gen 45:1) he unequivocal states that they should bring his father to Egypt immediately.
1) Could there be an intervening period between this time & the bringing in of Jacob to Egypt to have warranted Benjamin to have sired ten sons
2) Or could Benjamin have had sons while he was still called a lad?