i always let my stim kids stim out.  usually around the outside perimeter, and i explain to them individually that stim is ok but hitting/crashing is not.  imho, trying to control the stim only makes it worse.   let it flow, dont let it build up, use it wisely, and exercise often.

http://link.springer.com/article/10.1007/s10802-015-0011-1?no-access=true