COMPOSE IT - Unlock your inner Bach (advanced)

If you have a budding Bach on your hands who is more advanced, or if you want to help your children notate songs that come into their head, Noteflight is a program that allows for fairly advanced composition in its free version.  It's simple to get started and is a great activity for stimulating learning to read music.

noteflight.com

Also See "COMPOSE IT - Unlock your inner Bach"