Beta reading should be done to a peaceful place.
At least, if you're reading great literary works like, I don't know, such peaceful (read: boring) books like as Moby Dick. (I have a thing against Moby Dick, although I do give props for best first sentence ever.)
If you're reading middle-grade, then it makes perfect sense for your brother to be throwing pencils into the air so that they'll hit the fan and hopefully/hopefully-not (depending on whose side you're on) fall into your water.
Beta reading in my world. :)