Well his daughter the younger one i think leads the writing staff of clone wars. The other one a sort of famous mmo fighter.
His son not sure if he will ever be a director.
Spielberg's kids what have they done. Never heard of anything except his son urging indiana jones IV and look how that turned out.
The possibility of Lucas kids directing or writing the next star wars film series is probably non existent considering its probably written into his will their will be no more than six films and that the oot remain buried.