Playing melodies on the black keys is a good way to introduce a young, or older learner, to the keyboard. In addition to playing familiar pentatonic tunes on the black keys, it’s good to invite the student to improvise, to make up his/her own tunes.
It might be helpful to relate this to a G major scale. The pitches in parentheses are omitted: G A B (C) D E (F#) G.
The 3rd and 7th pitches of the scale are omitted: 1 2 3 (4) 5 6 (7) 8; DO RE MI (FA) SO LA (TI) DO