Growing Mouth Bacteria in a bowl of sugar water
I like the concept of this demonstration for teaching science!
As rotaredom's comment pointed out, bacteria does not grow in plain water with sugar. Bacteria require a nitrogen source, trace minerals and vitamins to grow in significant numbers.
I have a few recommendations that you can try, but I attached an extensive guide to kitchen microbiology below.

Growing the microbes in diluted milk (~ 1:10 dilution) + salt (a pinch for half glass) with and without added sugar (1 teaspoon for half a glass) might be a good starting point. Add yeast extract if you have some (not bakers yeast, but this is the killed version of that).

Make sure to boil and cool the milk, the container and other ingredients to keep conditions sterile.

Avoid a cotton swab if you can (as the cotton might not be sterile), I would take some milk solution, swirl it in my mouth and spit it back into the same container.

Also have a sterile culture in the experiment, without the oral microbes for comparison and a great teaching point.

Grow the microbes for 2-4 days with an air permeable cap (the air allows faster growing aerobic organisms, the cap ensures dust does not fall in)
Extensive source: Microbiology at Home: A short non-laboratory manual for enthusiasts and bioartists
PS: I'm studying PhD to be a microbiologist