USB Microscopes
Solution 1:
Without going through and listing X number of Microscopes I can tell you that most USB microscopes are simply cameras attached to lenses. Checking through the Hardware Support section of the Ubuntu Wiki also yields no mention of Microscope support.
That said your best option is to find 3-5 different models you think are good for your uses (and for your children) then search for that specific model (or just brand) with either the word "Linux" or "Ubuntu" at the end. Doing so will give you some feedback as to it's compatibility.
Solution 2:
No sure how many work but there is Celestron-44302-Handheld-Digital-Microscope on amazon:
HERE is one good user review.
Also there is a Intel Play QX3 that has supporting projects for Linux. See HERE for more info.
good luck!
Solution 3:
I've seen positive things about the Veho USB microscope under Linux. It appears to behave like any other UVC-video-compliant device in that it should "just work" these days. It should present as a video input device so you should be able to use it anywhere you'd use a webcam (Cheese, Flash online, etc).
And they're only about £30 so not a disaster if I'm wrong ;)
I suggest you find a model and then stick it in Google with "Linux" on the end to see if other users have had luck.
Edit: It seems like Veho and a dozen other companies just stamp their brand on this fairly generic hardware. For example, here's one by DigiMicro on DealExtreme for $30 including worldwide delivery. Note that this comes from the depths of China so a) it'll take a few weeks to get delivered and b) it won't be worth sending it back if it fails under warranty... But it's slightly cheaper.
Solution 4:
Camorama works for me, similar but different device from China though. You can install it with the command as follows.
sudo apt-get install camorama