Every good scientist knows the importance of a detailed lab journal. I learned the hard way as a graduate student when I needed to go back several months, but my notes were very poor. It resulted in several days of repeating work I had already done, leading to a lot of wasted time. My 8 year old has recently started a science journal to write down a few basics of each science experiment she does. This was her idea and although she doesn't write down a lot, she likes to draw pictures of the experiment and write down the results.
A lab journal is a great way to introduce kids to real science. The older your child, the more detailed their journal should be. Not only should a journal detail the procedure, it should include any data, along with results, and a discussion of results. Did the results make sense? Why or why not? What went wrong? What went right? What would you do differently? How might you tweak the experiment to test something different?
As a scientist and a teacher, I definitely encourage the use of scientific journals in any experiment.