What is the simplest way to demonstrate the germicidal property of UV light?
You can make homemade bacterial growth medium.

https://www.madaboutscience.com.au/shop/science-extra/post/grow-bacteria-on-homemade-agar-plates/

Basically it is just nutrient jello: beef stock, sugar, gelatin to make a firm surface.  It would be nice to have all your cultures in uniform plates or cups.  You could order petri dishes or use what you have available.  I would consider an ice cube tray or you could cut off disposable Red Solo cups to make dishes.

Then forget e.coli.  Inoculate the dishes with random things in the environment.  Everything has microbial life.   Fingerprints from various people are fine. Tongueprints would be a good variation - the jello is not poisonous.   A paw print from an animal would be fun.  A stick from the yard.  A snail.  Label each one with a label you can see from above - maybe a piece of paper on the table next to it.  

Each of these prints will grow a mixed bunch of germs and fungi.  You might want to keep them cool because they will grow fast.  Take pictures periodically over time.  Keep them covered so they don't dry out.

Each type of dish (for example, Ted's tongue print) will have one dish in the dark (under foil) one in the light and one under UV light.  If you make big dishes you could have a half and half: light side and dark side.  

Count colonies that can be seen at each time period.  That is your hard number and you can graph it over time.  The lines will diverge which will be a great visual.

In addition to the inhibitory effect of UV you can discuss how it differs.  Suppose there are orange colonies which do not seem to be inhibited by UV in the same way as everything else.  You could smear some of the orange on a microscope and look it is.  Is it some resistant organism?  You could discuss the variety of colonies that appear depending on the inoculum.