Me again, with more logic issues from "Alaidia."  

  This time, I am wondering how some young boys can get in enough trouble that one of them ends up with a scar - without the accident being too gruesome.
The set-up is thus.  My three MCs - Nathan, Peter, and Mark, all royalbloods - are gathered at the palace for a ceremony.  They are either in the range of 5-9 years or 9-11 (still undecided at this point).  Before the night is out, Nathan's royal parents will be killed and the three boys will separate for 9+ years.  Before that happens, however, I would like them to get into an accident while playing that leaves Nathan with a rather large scar on his wrist.
I think this could happen easily enough, especially since poor medical care of the era can make a scar worse.  But I haven't the foggiest idea what kind of trouble the young boys could get into.  I don't want it to be horribly graphic, whatever it is.
Ideally, the accident would be Peter's fault.  Even better - Mark could be picking on Nathan, and Peter, while attempting to rectify the situation, makes it worse.  That will foreshadow the boys' relationship as adults.  It would also be appropriate if the accident tied into the chaos following the murder and revolt.
So the ultimate question is - how can my young protagonist hurt himself?  The society is colonial with medieval politics, if that helps.