I say go for the Digital Blue QX5 microscope, hands-down. Here's why.
We've had two optical microscopes and a hand-held magnifier available to the kids. A small Bausch & Lomb (i.e. high quality optics) 'scope we got off eBay for about $30, list price around $90, was the first purchase. It had a single 50X objective and the quality of the image was good. But it was small and fiddly to focus and the illumination was limited to indirect lighting. Plus we found that only our eldest could actually squint down the barrel and see anything. Our younger two (under 7 at the time) just couldn't reliably see through the barrel. It takes a fair bit of concentration and mental ability to train your eyes to see down a single barrel. They tried the hand-covering-other-eye trick (since they couldn't wink) but that didn't seem to help much, and it just left them unbalanced as they leaned over the scope, covered one eye and tried to adjust the focus with the other hand. The whole endeavour was just exhausting for them and frustrating.
We have brought home the medical clinic scope on a number of occasions. It's a professional quality scope with high magnification (suitable for micro-organisms like bacteria and yeasts) and an oil-immersion lens. Large, sturdy, great lighting. Our eldest saw a few interesting things with it, but the younger kids had the same "squinting down a tiny hole" challenges. Sometimes Noah (then 6) thought he could see something. Sophie (then 4) never did. The depth of field at high magnifications is negligible, so slide preparation has to be done properly (thin samples, maybe using a microtome, slide and cover slip, etc. or else just buying and using prepared slides). What happened was that dh and I spent 20 minutes in slide preparation, slide-gazing and fine-focusing, and then we'd get the kids to peek in and see what we'd found. It definitely didn't maintain child-led enthusiasm!
We gave up on the medical scope (will use it again when the kids are teens), sold the little Bausch & Lomb and I bought an inexpensive hand-held magnifier (15X magnification, I believe) and they seemed to like that more. With the lower magnification it had a large depth of focus and it could be brought to the eye, rather than vice versa, so my then-6yo was able to use it reasonably well. Also, at low magnification, things bore more resemblance to their unmagnified images, and that somehow made the viewing "realer" and more exciting for the kids.
I still had microscope dreams for the kids, so I did a lot of reading about kids & scopes. I decided that if we could afford to be a microscope at some point, it would probably make the most sense to buy a high-quality binocular dissecting microscope (i.e. relatively low magnification, two barrels to look down for relaxed viewing, large depth of field, easy-to-use focusing knobs). But these guys are $320 minimum. So we waited.
Then I stumbled across reviews and images from the Digital Blue. I was really impressed. So we used Grandma's Christmas-and-birthday-money-for-the-year to order one. We bought a slightly older version on sale and got it for about $50 (the best price I've seen lately is about $100). We're really happy with it.
It has low to medium magnification (10X, 60X and 200X). The barrel can be removed so that, like a hand-held magnifier, it can be used to examine things you can't easily fit on the stage -- fingerprints, eyeballs, the winding on the strings of a violin, the surface of a rock, etc.. There is none of the exhausting squinting down the barrel -- you're looking at an image as big as your computer screen. At low magnification the depth of field is vast and focusing is dead easy. Medium magnification the kids can use easily with a bit of practice. "High" (i.e. 200X) magnification focusing is of course fussy (that's just the nature of the physics) and it is difficult to get sharp images without doing real glass-and-coverslip slide preparation, but it is possible with patience. Using the scope is no longer a solitary taking-turns endeavour ... three or four kids +/- adults can watch in real time as something leaps into focus. They can point on the screen ("what's that?" or "move over this way"). The image is not inverted as in a regular scope, so moving around the image is easy and intuitive. We can capture images to save and share (later, with daddy, or with a friend, or to insert in another document like a report or newsletter). We can save video clips of little critters in motion. It's like the difference between remembering your holiday and going through a photo album. When you can retrieve photos, everything stays more real and present in your memory.
The microscope software installed beautifully in a minute or two with no hitches in both WindowsXP and W98. The microscope has proved sturdy enough for our use. I don't worry about supervising, and sometimes the barrel gets treated a little roughly -- no problems so far. It continues to interest the kids. While is does run through the computer, unless you are capturing and exporting images, you don't have the sense while you're using it that you're "using the computer". All the activity is at the microscope end, with the computer screen simply being a real-time video feed of what the scope is seeing. In other words, your hands can stay totally on the microscope and your specimens, and you don't need to fuss with the mouse or keyboard except for, say, adjusting lighting off the "auto" setting, or viewing previously-captured images.
Really I can't say enough great things about this microscope and its developmental suitability for kids.