Celestron makes a $50 webcam-like (USB) eyepiece camera for telescopes, but it works nicely with microscopes as well. Celestron also sells inexpensive mechanical microscopes.
For thin specimens or slices, you want a compound microscope. For stereo images, you need a stereomicroscope (a.k.a. dissecting microscope). The two have very different designs. The stereomicroscope has two identical objective lenses next to each other, like the two lenses in a stereo digital camera. Unless you need very high magnifications, you can do pretty well with any of the "toy" USB devices that has a built-in webcam and a single magnifier lens (e.g. RadioShack zOrb for ca. $40).
You can build your own compound microscope, which might make a great project for a 7-year-old: http://www.funsci.com/fun3_en/ucomp1/ucomp1.htm
Finally, you can make a Leeuwenhoek microscope (the original microscope design, from the 1600s) with a single spherical lens of 2-3mm diameter and some ordindary household materials. You can make the lens if you have a torch or Bunsen burner, or get one from Edmund Optics (edmundoptics.com). Lots of how-to sites on the web, e.g. http://bizarrelabs.com/micro.htm (one design on this site uses a drop of water as the lens).
Sounds like a fun project.