I’d be looking at reflective stickers, if your main concern is visibility.
I have a separate thread for general bike advice for people who are holding back. It’s in IMHO.
I don’t get it. 24" or 26" is the diameter of the entire wheel. If you put a 24" wheel into a frame designed for a 26" wheel, then the attachment point for the brakes will put the brake blocks exactly 1" too far above the rim. I’ve never seen brakes that have that much vertical adjustment. This applies both to the front and rear.
One more bit of tandem advice, it takes a bit of coordination, so we always said “Stop pedaling” when we had a rest otherwise the “stoker” gets their feet knocked off the pedals when the “captain” stops pushing.
Yes, you can. Those same kinds of paint are used to spray-paint all sorts of metallic and plastic substrates. You may want to check if the one you’re looking at indicates a “best substrate” (plastic, steel, aluminum), but the people working on formulation spend a lot more time finding the right shade than on anything else (specially for automotive, which has to be exact shades - people painting yellow metal poles usually don’t care so much).
The one I recently put on my curb-sale bike has about 3cm of play, which is more than an inch, and it sits lower, so it has to be adjusted all the way up, instead of about 1/3 of the way down, where the original brake was. It’s a side-pull BMX brake, which I bought because I could NOT get the original center-pull adjusted correctly. Because of the way the bike is made, and the fact that I wanted a carrier on it, and some other side-pulls were very expensive, I took a chance that I could fit this caliper, and it worked. I have measured, and the same caliper will work on the tandem frame with a 24" wheel. It’s a very good caliper. Even on a slight downhill, my bike will stop on a dime using only the rear brake.
RE: pedaling and stopping. Good advice on warnings. Fortunately, my son knows just a little sign language. We’ll talk about signing “slow” and “stop,” and how he needs to pay attention to the road too, to watch for hazards that might require a sudden stop. I will consider light pedals for the rear.
Ah, I see - I’m not familiar with BMX stuff, but I just Google imaged for BMX side pull brakes and I see you are correct.
Google an image of a BMX. They have fat tires and extra clearance between the tire and the frame. That’s why the brakes are designed the way they are. I couldn’t go any smaller than 26" –> 24" I could make a three or four inch swap, without going to disc brakes, but two inches is doable. Albeit, if I planned on using this on the street, or on high hills, I’d consider a back-up disc brake on the rear wheel.
For further non-paint discussion, here’s my bike advice thread.