Rather than creating an app, Noteflight developers put their time and efforts into creating a version of their site running in HTML5, that mobile devices could run.
They claim it is ready for Safari and Chrome, with other browsers soon to follow, but I have used it successfully with many browsers!
Open up your browser to www.noteflight.com and check it out!
Look at that picture!! Choose the rhythms, and touch the note you wish to play on the keyboard. SO SIMPLE!!
I applaud the Noteflight developers for making this a reality!
In the mean time, have fun!
(By the way-- don't steal my masterpiece up there that I created in 30 seconds flat. Okay? Ok.)