Hello - Are your workshops to be limited to music and music-making, or are you looking at a wider range of activities ? If the latter, then here are a couple of books packed full of games and activities just right for this age-group:
"100 + Ideas for Drama" by Anna Scher & Charles Verrall (of the Anna Scher Children's Theatre) publ. Heinemmann ISBN 0-435-18799-6
and
"The Drama Ideas Bank" by Mary Brooks publ. KCP Publications ISBN 1-84321-030-4

If you *are* looking for things not-just-musical, I can add some more specific sessions here that have been successful and fun.

- jeanie