Neither of them has ever seen Erebor at its full glory, from the top of the mountain to the deepest mine. They listen to Uncle's stories, to Mother's songs, and they create new tales, starring themselves in those halls they've never been to.
They follow Uncle on his quest, though Mother tries to forbid it - but they are nearly of age, and heirs of Durin, and Mother nearly takes Kili's place, until Uncle says that one of the line must stay back, one who could lead, should the worst happen.
At that, Mother nearly took her axe to Uncle, but she at last stood aside.
(Fili has Uncle's commanding presence; Kili has Mother's charm. Neither of them has the experience to lead, should the worst happen. But they are strong, and they are quick, and when they fight together against all comers, they are nigh unbeatable.)
Neither of them has ever seen Erebor at its fullest glory.
Neither of them ever will.