Image remains blurred irrespective of focus, because no extra information is available compared to a smaller, sharp image. For ordinary optics, the limit is around 600-800x without use of oil immersion.
The USB attachment is cool. Especially if you have a "honking big" monitor. At the lab I occasional work at, they have a 40 inch monitor for doing examinations. Put the sample under the scope and the image on the screen. You can sit up comfortably and have the image wide-screen in HD. Get something you like and BLIP capture it. Probably a high quality USB is the choice for home labs.
Also interesting is the mount that allows you to put your phone on the eye piece. It converts the scope into a projection scope. The camera on many cell phones these days is quite good. That might be interesting for the kind of person who wants to carry the scope out to the woods or the pond or whatever.
I have tried several optical attachments for smart phones so they can function as microscopes.
They have generally been disappointing.
Recently, I got my daughter a digital microscope that can either plug into a computer via a cable or connect wirelessly (bluetooth) to a computer or smart phone.
This worked well, was easy to use, and was of a similar quality to that of a corded consumer digital scope.
It is pretty handy (my daughter is doing some work as a field biologist) and portable.
She showed her boss the scope who then decided to get one. Here is a link to it on Amazon.
Lately, I am more active in a german group on microscopy than in PF. There are also lots of people present who are active in education. The general consensus there is that for children up to maybe 10 years it is better to start with a binocular lens instead of a real microscope. The reasons given are the following ones:
1. For children, it is difficult to relate images of high magnification to the objects they were taken from.
2. The objects they are interested in, like plants or insects from the garden, can be viewed without having to kill them or without needing involved sample preparation.
3. In higher magnification, the field depth is very small so that you need to change the focus constantly to get an impression of the 3d structure of an object. This is difficult for children, or given a reduced span of attention as compared to an adult, they may loose interest, before dominating the technique.
Concerning the choice of a microscope, nowadays there are second hand microscopes from top enterprises on the market, like from Zeiss, Leica or Olympus, for a price you would have to pay for a new toy microscope of low quality produced by a noname company in India or China. I would always prefer these to a fashy looking USB microscope. If you really want to watch things on screen, get a cheap adaptor for the smartphone. Children tend to have less problems using a smartphone than a microscope, these days.
So many excellent solutions to a wide variety of questions. If the OP is not a teacher then I suspect he/she is a grandparent or equivalent. I am convinced that what's needed is something robust enough to use out in the field, carryable in a pocket or small pack and non electronic. Small relatives need results fast if you want to keep them interested. I have noticed that the 'quality' factor that adults are concerned with is way down the list for kids. My grandkids play very happily with my old Pentax binos which are well out of collimation; I cannot use the things. Same with using astronomical telescopes. They are looking for different things from adults and the need to be in their teens before they are prepared to deal with the fiddly bits of hi tech equipment.
The microfiche reader would have been great a few years ago (I would have loved one at school) but classrooms tend now to have video projectors and big screens so a USB microscope would be easy to store and hopefully the teacher would be familiar with the idea of using the app - and its filing system.
As always, it's horses for courses. The age of the kid is a major factor.