This is by FAR my easiest subject to teach, and it's my boys favorite! YAY. We are doing a spectrum of grades, but I'll seperate it by 7th grade (my eldest) and 5th grade (my youngest.)
Each child will have a notebook they must maintain as their Science & Experiment notebook. THat way whenever they learn something new, or try a new experiment, they have one centralized location to keep it in.