A First Aider is medically trained, and if the kid had a broken arm, all the more reason to get involved. (Edit- I see this was aimed at the RTA not the first post, still largely relevant though)
The only real concern for a First Aider is not to give any drugs or creams except the patients own Epi-Pen if you’ve been given the (very basic) training, and to only treat the patient if they agree. If they don’t agree, you just wait until they fall unconscious and then you can crack on, it’s your duty.
I don’t think anyone has ever been prosecuted for causing further injury when they had good intentions at heart.
It’s a 1 day course (or 3 for a bit more detail) to become a First Aider. Anyone who would not intervene just in case there was a serious injury should get themselves on one (when things get back to normal)