OK, slightly wrong sub-forum, but I can fix that for you.
I am pleased that you have a daughter so excited for science. The test you have chosen is actually pretty simple to test and doesn't really involve a microscope, but is still very visual and easily explainable...
What you need are some growth medium plates - these don't need to be anything special, LB medium is a good general medium, otherwise Blood agar is a good choice. I don't know where you can get these on the open market (try asking the microbiology department at a local university/college or even the school - they may be able to give you some), but basically these are a bit like jello with some added nutrients - you can buy gelatin and use some beef-stock (see here) to make some basic ones. You may need a fair number of these depending on the number of conditions you want to test.
You should also have some clean/sterile swabs or something similar - these can often be found in first-aid kits, or try your local pharmacy for sterile gauze.
For the test: I would take a sterile gauze (wear disposable gloves so you don't contaminate it). Drop it on the floor. After 3 sec, pick it up and wipe it on the growth medium plate and see what grows. You can also test this with a wet gauze (use sterile water (bottled is usually fine) or sterile saline solution) and look at the difference compared to a dry gauze (this is the equivalent of the difference between using the 3 second rule on a cookie or chip compared to an apple slice).
You could also test whether blowing on the dropped gauze makes any difference, time it sits on the floor. You may also want to see what happens when you pick up the gauze with bare hands (or just put their hand on a plate, maybe before and after washing hands). You can also test different surfaces (e.g. carpet vs wooden or tile floor, kitchen bench, bathroom...etc).
You may find that you need to dampen (use sterile saline) the gauze before wiping it on the plate, as this will help transfer any bacteria. You should also have a "control" where you put a gauze directly on the plate to test the sterility of the starting conditions.
Most of the cultures isolated will be non-pathogenic, however they should all be treated carefully - make sure you dispose of them carefully, and don't go opening the plates too much. Always wear gloves after inoculation and thoroughly wash your hands after handling the plates.
Bacterial colonies will appear as mostly cream-colored dots, any orange, bright yellow and red ones are likely to be yeasts, and fuzzy/hairy ones are often fungi/molds. If you do want to look at them under the microscope to see the different types, most of the bacteria will be hard to see without staining, but under the right conditions (usually need 1000x magnification to see properly). Yeasts are pretty easily visible at 20x mag, they will look like balls or eggs possibly with smaller versions budding off the side of them. Fungi/molds are easiest to see if you use some sticky tape and gently touch the top of the colony, then use this like a cover-slip. Fungi are big, so don't need much magnification.