I believe one of the tricks to keeping them interested is allowing them to play music they enjoy. Sometimes they'll ask to learn a song off one of their favourite shows - or something they have seen on tv or youtube. Luckily the music I haven't been able to track down, hasn't been too difficult to figure out for them (so far!)
One of the songs Lani has loved learning is a modern cello/piano duet called 'wind.' I orginally downloaded it to learn it myself to play with a cellist friend. Lani heard me play it once and loved it. She decided it would be her next song. I explained that we'd have to play it together as it is a bit tricky for her two little hands :) She loves to play together anyway, so she was very happy with that idea.
I think she has done a really good job keeping the melody line flowing throughout while I play the left hand - especially as this particular piece is in 5/4 - A difficult time signature to keep. Lani has been blessed with an innate sense of rhythm which means she can handle the clashing rhythms between my hand and hers - it also makes my job of teaching her much easier.
I love her face at the end. She is so cute.