Now the way the Hebrews counted included the first generation. So this is simply his grandchildren, not his great grandchildren.
Hebrews 11:10 speaking of Abraham says, ‘For he looked for a city which hath foundations, whose builder and maker is God.’ He looked for the heavenly city, and the writer of Hebrews goes on to spell it out in detail. ‘13 These all died in faith, not having received the promises, but having seen them afar off, and were persuaded of them, and embraced them, and confessed that they were strangers and pilgrims on the earth’, and then verse 16, ‘But now they desire a better country, that is, an heavenly: wherefore God is not ashamed to be called their God: for he hath prepared for them a city.’ There is no doubt that, like Abraham, like Isaac, and Jacob, Joseph believed that the land that they would be given on earth for a season was only a prophecy in its turn of the eternal new heavens and new earth which the people of God would dwell in for ever. That’s the great promise.