They do exist. They're not dangerous to you or your son, they're actually just interested in the bow hair. The trick to never having to deal with them, and a pretty good indicator of how often kids are practicing, is to keep the violin case open and exposed to indirect sunlight on a regular basis. If your son isn't going to be practicing for more than a few days or so at a time then it's always a good idea to take the bow out of the case and keep it somewhere high and out of the way so that it doesn't stay just in the darkness of the case.