I suggest that you give details of the iron (and tip) and solder that you are using and a close up photo of the wires that are being problematic.
The iron temperature control may be faulty and the iron just not getting to soldering temperatures. Or you may have it set too low.
The thermal mass of typical USB wires is so low that, if the solder actually melts freely at the tip end when not soldering anything, it should do so when soldering these wires.
Get them working on their mathematic skills, instead. You can give them a really good head start - mine could solve simple differential equations at age 11.
I’d suggest peeling off just one strand of the wire and use that. You will need to insulate the wire from the nail - Wrapping paper around the nail will do. You will need to insulate the wire from itself, so that adjacent turns don’t touch. You can just space the turns out along the paper sleeve. That should do the trick. If you have twine, you can wind that on with the copper wire and use the twine to keep the turns from touching each other.
Oh to straighten copper wire, if you don’t know this trick - hold your nail in a vice and wrap a single turn of copper wire around it. If you then pull on one end of the wire, keeping a little tension on the other - with a bit of practice it will give you a very straight end result, as the wire pulls away from the nail.
The standard way of looking at this is to consider a capacitor-resistor series combination going to ground. Connect a 10v (wrt ground) supply to the capacitor and the voltage across the resistor rises to +10v, then decays. Now connect that capacitor to ground and that same resistor gets -10v across it, which then decays. Whatever is connected to the capacitor “top” terminal has to be able to sink current as well as source it.
That’s what generators in simulators do - they have zero internal impedance (usually). They sink currents as well as source them.
On a KISS basis - I tend to just use a bimetallic switch or omit the temperature control and just run the fan from power up. It’s possible for a processor to suffer some non-handled exception where it no longer executes the temperature management routine.