When children blog all in the same area, this could be available for the parents to see their work firsthand. I think that blogging enhances the lesson because if there are children that do not want to raise their hands and "speak" in class, would most likely write more in depth and share more on the blog vs in the classroom. I was one of those children. I would much prefer to write something than to publicly speak in front of anyone! Since then, I have overcome my fear of raising my hand, but it might ease children into it!
The grade I would use it in would be 4th grade, and I would give the children a writing prompt. Starting with something easy to get them used to the blogging atmosphere like what they would like to be when they grow up and why. I would then have them respond to a certain number of classmates, commenting on their career choice. I feel this would be something that each child could relate to each other about. I think every child has thought at one time or another about what they want to be "when they grow up." My rubric would consist of how well they made comments on each others post and how well their writing was. I have always had children that could explain themselves better with writing, than have to answer multiple choice. This is where the future is going, and I believe that the sooner they are exposed to it the better.