And btw I'm pretty sure you DO have to change brake fluid if it's contaminated. And bleed the system to eliminate air pockets.
Yes, if the fluid gets air bubbles (often the result corrosion leading to moisture in the line) it makes the brakes feel spongy and reduces the vehicle's stopping power when the brake pedal is pressed.
That said, flushing and/or bleeding a brake system is a very intense process that almost nobody would decide to do themselves unless absolutely necessary.
First, brake fluid is highly toxic, and must be disposed of at a toxic waste disposal center. It will destroy any surface it comes in contact with (like the painted surface of a car), so it's highly doubtful that a group of girl scouts would be allowed to handle brake fluid, especially anywhere near an actual car.
Second, a small amount of various substances (like grease or oil) getting mixed with brake fluid can permanently ruin a hydraulic brake system. Again, highly doubtful a group of girl scouts would be allowed anywhere near an actual car's master cylinder.
Third, to bleed a brake system (not to be confused with
flushing, which is the actual process of changing brake fluid), one would need to access the brake bleed screw on each brake, located on the inside (underbody-facing side) of the caliper (or cylinder for drum brakes) using a bleeder wrench. That means jacking the vehicle 4 times (once for each brake). Then someone has to sit in the car and press the brake pedal while toxic brake fluid shoots from the bleeder nozzle, which must be contained by the person underneath the car. If air gets back into the nozzle before the screw is returned, it can damage the brake system. Again, highly doubtful a group of girl scouts would be permitted anywhere near this process, nonetheless participate in doing it on an actual person's car.
Lastly, flushing a brake system is done using commercial equipment (using vacuum pressure), which is why you would take it to a qualified mechanic. I can believe that a group of girl scouts might have gone on a trip to an automotive repair facility and been told about this process (perhaps a badge-earning outing one afternoon, wherein the troop leader had to take her car in for servicing) and therefore learned that it is done by a mechanic, but to characterize that as having "practiced how to change brake fluid" would be quite the liberal use of the word "practiced." Using that definition, my wife also practiced how to change the oil by taking the car to Jiffy Lube .
