Hey ! I like your interpretation of the theme ! I love the little story and characters.
Even tho I am a guitarist (A mediocre one mind you...) I could not replicate the tune to pass the level. I think it is duable but maybe at more advance level. The firsts level should be really simple and short to give player time to adapt to the instrument and feedback on how it works.
It's still a great idea with some wholesome potential.