This is an plain old ice crystal. Welcome to the microscopic world! When water evaporates, high-energy molecules escape, leaving low-energy molecules of water. The droplets cool this way, and they freeze - the dirt or debris in the crystallizing droplets make curious patterns. Here are some in my samples I made last week, for comparison:
How about a close-up?
Here's one I made by accident, this time featuring a fluorescent dye.
Notice how cooling droplets never freeze the same way twice. You'll never find two identical crystallized droplets!
If you want to avoid these, prevent evaporation by putting nail polish or glycerol around and between your coverslip and slide, so that water cannot escape.
OFFTOPIC: You should also encourage your kid to try make some of his/her own crystals. You can use food dyes to make beautifully patterned art. Good moment to teach your kid about nucleation; why water doesn't freeze at 0° C, but rather at temperatures that tend to decrease as the volume of the water decreases and as the water impurity increases. And maybe sneak in a funfact about how the freezing of small water droplets into ice is an important process in cloud formation.