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Start with composition of simple melodies using the notes they have learnt on the piano - anything goes. I don't think they would be able to write specific "jazz" riffs without direction such as giving them the pentatonic scale notes only to compose from.
Once they know about chords you could then give them a simple chord progression to improvise around, eg start off with I and V only eventually leading to the blues chord progression. Then you could introduce the blues scale for the melody which gives more exciting tune possibilities.