I found this discussion on the Ning Education 2.0 site about blogging in the lower grades. If you're not already hip to that network, it is a rich rich resource. Unfortunately, it's so rich it almost discourages me from making my own wheel!
It was there that I found classblogmeister, a blog host designed with teaching in mind. May be worth exploring and signing up for. You have to contact the founder, educator David Warlick, to get a school code and such for your blog.