The reason it is important to have a repertoire ready to play (even if the repertoire is Mary Had A Little Lamb and Twinkle Twinkle Little Star) is because it is satisfying to the player as well as any listener; and in addition, it keeps the player motivated to continue learning new music. Think about it--if you've been studying architecture, baseball stats, finance, ballet, or any other activity, the more you build upon your foundation of knowledge, the more excited you are to keep studying. In music, the way we keep our repertoire list fresh and ready to play, is by reviewing the pieces we've already learned.
Enjoy the repetition of your songs that you've worked so hard to learn by choosing one or two pieces to review each time you practice.
Notice how you glide right over that fingering that used to be difficult and can direct your thoughts to the phrasing and musicality of the piece.
Luxuriate in the wholeness and beauty of the music. You have earned the privilege of conveying the composer's wishes because you've mastered the technique of the piece.
If you are helping a child review their pieces, the above concepts will be second nature to them. Watch as they musically blossom after reviewing their songs over many months and years.