Jocelyn, what a great idea!
I worked with first-grade teachers in Phoenix several years ago who had their students start blogging on classblogmeister, which I REALLY like as a platform (though it's not very showy). I think those particular blogs are long gone now, but first-grade teacher Kathy Cassidy has her students blog on the same platform -- it's pretty cool to see students' growth throughout the year. (The main page is Kathy's blog -- click on any students' name on the right-hand side to see that student's blog.)
I've recently tried out kidblog. It's easy to use and seems like a good platform for younger students, but I don't have extensive experience with it.
For more info, I wrote a blog post about blogging with students a while back that may be of interest to you -- it includes research showing that blogging improves students' writing skills.
I love tiwducate...very teacher controlled..no sign up required and very simple to use.