I would go for something like this, or equivalent. It offers up to 400x magnification, is simple enough for a child to use, while of sufficient quality not to outgrow in a hurry.

Laboratory Biological Microscope (item #44102) / Celestron.com - Telescope | Computerized Telescopes | Microscopes | Binoculars | SkyScout

If budget alllows, and she keen on the idea of microscopy, the next level of microscopes offers approximately the same sort of magnification, but comes with an electric bulb as the source of illumination, and a mechanical stage, so one can hover over the sample by simply turning a two knobs. By not having to touch this slide, the motion is a lot smoother. These features are not essential, IMO, but adds to the enjoyment.

Mintaka