I tried a wiki this year with my 5th grade library skills class -- we created a "Virtual Zoo." It was a big experiment, and the kids really liked it. The only trouble I had was with some student content being accidentally erased (as several students were working on pages at the same time). I fixed this problem by creating a page per animal, instead of having a "mammals" page with several kids working on it at once. I used the educational version of wikispaces and found it relatively easy to navigate.
Good luck! It's fun to try something new, even if it doesn't always work perfectly the first time.