Kindergarteners compose in ABAB form. 
They have 4 measures in 4/4 time for 16 beats.

Here is a slideshow of their composition papers. AMAZING work!!
You can pause the show and read their music.

The process is the same with the grade levels for writing the rhythmic composition with form, but older students have more rhythms and sometimes longer compositions.

After students have finished their composition as described in the ScreenCast, they share it with others to get feedback and help with errors if they need it.

Here is video footage of 1^st Graders and Kindergartners who are sharing their music.  The 1^st Graders are sharing their work and checking for mistakes.  The Kindergartners are group sharing and the person who wrote the music gets to play it on the drum while their classmates read it with them.

You can compose at home with the same worksheet!