Here are some 6th grade boys performing their own made-up gumboot dance.
They don't have gumboots, but that's not stopping them.....

You asked for more links for younger kids, this is the best I can find.  Look to the right-hand column.
I think for younger kids, it would be good to do something with a steady march and a sequence of maybe four moves that repeat over and over.

You might have a series of breaks where individual kids improvise after they have learned a vocabulary of sounds.

Basic Gumboot steps:

http://www.youtube.com/watch?v=p0nb5_wDR_M

http://www.youtube.com/watch?v=a0z1OO-qFHI

http://www.youtube.com/watch?v=ZUtQg7jVlUI

Group Dancing:

http://www.youtube.com/watch?v=U0Q51WVrR40

http://www.youtube.com/watch?v=6WdhA_i8YT0

http://www.youtube.com/watch?v=A5Nk-VdDuv8&list=PL7C4C182A9318E86C

More videos to learn from, use these with younger kids maybe.....

Gumboot dancing step 1
Gumboot dancing step 2,3
Gumboot dancing step 4,5,6,7