Hi,
I also teach students (11-18) in a percussion group, and I´m quite amazed that your younger pupils already play a decent tumbao, because the basic sounds are hard to learn I Think.
Well, the next thing you could start could be either rhythmic improvisation, of course or teaching them every rhythm from bembe to funk..
My experience is that especially the younger pupils have not gathered enough rhythmical understanding yet so that they have a masterly overview over what the basic rhythm is or what they should play when improvisiing. NEVERTHELESS, I always try to give them simple patterns to memorize and then to play small variations on it.
Cultural heritage is one thing, but didactic reduction to what can be played is another. You can´t teach little Europeans to improvise on a quinto in rumba like ####..
To me (+my students) the motivation focuses on fun in playing and having small gigs..
As far as other ideas are concerned Latin Percussion has some ideas on its site for younger children, and another link I recently found is
http://www.playdrums.com/drum_circle.html
But that is probably not suitable for your age group.
ood luck and welcome on board anyway1