In short
The sons of the kingdom
are the Jews of Jesus’ own day who are descended from Abraham physically, but not spiritually, because they have rejected Jesus (see John 1:11).
The glorious image of the feast in the kingdom of heaven in Matthew 8:11 is radically contrasted in Matthew 8:12 by the prophecy that the sons of the kingdom
will be thrown into outer darkness. While the believers from the Old Covenant is present in the kingdom of heaven, Jesus’ own Jewish contemporaries, who are sons of the kingdom
by virtue of their lineage yet who rejected Jesus (see John 1:11), will, on judgment day, be cast out of the kingdom.1
11 I tell you, many will come from east and west and recline at table with Abraham, Isaac, and Jacob in the kingdom of heaven,