I started a blog for each of my school age children under my blogger account and made them authors on the accounts. I did it this way so I didn't have to log in and out of separate accounts. They add reports, little bits of information about topics we are studying, make journal entries and they also like adding pictures that relate to what they are writing about. We also take photos of notebooking pages, lapbooks and projects they have completed and add those as well. They will often copy and paste what they wrote on their blog into a word processor and create a notebook page, dressing it up a little more then they would on their blog.
Blogger is great because they have lots of little gadgets to make this simple for a homeschooling parent. They have a gadget called "labels" so your child can add the labels at the end of their post and then every subject is in its own category. I LOVE this feature. I can look at all their writing assignments on one page, for example. Blogger is so user friendly this way and very easy for even young children to learn.
My youngest son, who is 10, is the most excited about his blog and adds at least 4-5 entries every day. Some of things we add to his and the other kids blogs are: typing, writing assignments, nature study findings, and scriptures we are studying. I will type while he narrates because after a few minutes his fingers tire. He types out scriptures he is studying and then also uses them for his copywork.
You can make your children's blogs private and only invite those you trust to read them. Grandparents especially get a big kick out of them.
Having blogs for our kids has been a great thing for our family. It is in a safe and secure place and they can look back on them for years to come.