I have been around lacrosse for 20 years ..many years in youth and now still in youth along with HS.
For a coach to say to anyone and especially youth " break his arms" is totally irresponsible along with frankly someone being a complete jerk. This isnt some common term used in lacrosse. I havent even heard it used as such? And to belittle a 5th grader ..does anyone even need to say anything about this besides it being completely wrong?

Their is a certain responsibility to coaching youth sports. Having a pattern of this behavior is not a good thing. The league should be proactive and give him a few games off.