React Toy Piano
Try it out at http://cvuorinen.github.io/react-toy-piano
Works best on a touch screen device with a large screen (tablet etc.). Add to homescreen to get rid of the browser UI.
My daughter wanted to learn playing piano but we didn't have one, so we decided to make one with code. She had already learned a few songs on a xylophone with colored keys, so we used the same colors for the piano keys so she could instantly play a few simple songs and really liked it.
We have added more songs afterwards every now and then, after she has learned all the previous ones and wants something new. Some of the songs might be Finnish origin and not that familiar elsewhere, but there should be few that are more international.