Those adapters should definitely be fine for 24 V. Running the fans off 19 V will probably work, but they will run at slightly slower RPM (probably not a big problem for a filter).
From looking at the LED bulb, I can tell you that it will not work very well in that flashlight.
The reflector of the flashlight is built so light coming from a very small source (like the filament of an incandescent bulb) is directed forward in a focused beam. With the led bulb, light is coming from 10 different spots, none of them being in the focus point of the reflector. The result will be a spread out beam that won’t be bright over longer distances.
The only type of LED bulb that could work is something like this car replacement bulb that keeps the light source to a relatively small spot. But I don’t think those are available in the size you need.
Not really. With that bulb, all the light is focused forward, not to the side. The light will never hit the reflector, it is only focused by the lens in the LED housing, and that isn’t enough.
As a young child, I had a wooden board with a bunch of different light switches mounted to it. Nothing to tinker with, but I had a lot of fun with the physical sensation of switching them on and off. I think that’s all you can really ask for at that age.
Later on, I enjoyed taking the switches apart, figuring out how they work and using them in my electronics projects.
My point is, whatever you choose should be physically fun, even without understanding it. A few switches and maybe a blinking light can entertain a toddler for a long time. The tinkering can come later.