At SFSKids,you can compose write you own music on a staff and hear it played back. You can choose a tempo, time signature, and instrument to write for.
With ToneMatrix, you can create patterns on a grid/matrix which will play your music with sound of bells.
At NyPhilKids you will find several games to play where you can make your own music using the sounds of the orchestra.
At Isle of Tune, create your own musical island, with streets, lamps, trees and flowers that make sounds as cars drive by them. Just click on objects to change the sounds to the way you like them.
Musescore is a program you can use to create and save sheet music. Choose which instruments you want to write for, and hit play to hear your music played for you.