MARIO PAINT COMPOSER is the program that I use to teach composition in Year 2. They can't get enough of it~~I can't wait to have them compose their own song when school starts again after the holidays~~

Mario Paint composer can be downloaded here:

Please note that the musical function is quite limited, I'd say it's only suitable for children under 7.

For older kids, I recommend "Garage Band" (default program from iMac).