Certainly by the time your child is doing high school level biology, you're going to need a microscope; I wouldn't wait until then, though. Originally I bought my microscope when I was teaching a high school biology for our homeschool co-op, but my children have gotten so much fun out of it that I would suggest you get one sooner. Getting out the microscope is a guaranteed afternoon of fun in our house.
Be sure to have plenty of slides and cover slips, available at Home Science Tools. One thing that make exploration easier for novices is prepared slides. The Complete Elementary Slide Set has a great variety of specimens.
Two great beginner books to go along with your microscope are Greg's Microscope and Janice VanCleave's Microscopes and Magnifying Lenses. (Enter micoscope* in the Search My Books at LibraryThing box in the right sidebar to display these books. You'll also see The Usborne Complete Book of the Microscope, which is visually beautiful but without many more home microscope activities than Greg's Microscope.)
A very simple (and smelly) activity is stuffing a handful of grass into a jar, covering it with pond water and a lid, and letting it sit for 3 to 5 days, making sure to open the lid daily. Put a drop of that water on a slide, cover it with a cover slip, and check out the active microorganisms.