Some common tips for teaching biology to kids are to keep it as hands-on as possible, and to acquire a pre-made curriculum. Find such curricula at websites such as csun.edu or scienceshepherd.com.
While it is generally good advice to teach biology to kids in as hands-on a manner as possible, the form this teaching takes depends on the age of the kids in question.
The most common time for students to take biology classes is the at high school level. For these ages it is useful to focus on reading, then use hands-on experimentation to demonstrate a well-considered selection of the principles being studied. Some common experiment topics that cover biology basics are animal regeneration, plant growth, bacteria growth, anatomy, cellular respiration and osmosis/diffusion.
In elementary years, it makes sense to focus more heavily on experimentation. In part, this is due to most students in this age range lacking the math skills necessary to tackle many scientific topics. Many age-appropriate experiments are available at ScienceKids.co.nz, and include topics such as bacteria breeding, smell and taste, microscopic creatures and lung volume.
Regardless of the age of your students, it is often best to use a pre-made curriculum appropriate for their age.