Practice laying down. The laws of gravity will help a violinist feel a naturally aligned state in their body when they play laying down. This is an incredible tool for teaching the neck to release, support the shoulders finding the base of their sockets and even getting the knees to soften up. Young children especially love this because it feels like a novelty however this practice technique is appropriate for players of all ages and levels. It will create a very awkward feeling in the bow arm and compromise the sound but the long term results are more than worth the short term compromises.
Tighten up shoulders as much as possible for 5 seconds and release. I tell my students to "put their shoulders in their ears." After this intense use of muscles the shoulders will respond with fatigue and rest by staying down in the bottom of their sockets. They may not stay down for long but this exercise helps develop the necessary awareness to address tight shoulders. In addition to creating a muscle fatigue, this also helps the student realize the extreme of an action so they have more control in choosing where their shoulders go when playing.
Evaluate chin rest and shoulder pad set-up. Not only do young students with growing bodies have different proportions in their neck-shoulder-arms approximately every 6 months but their playing needs are also growing. The chin rests and shoulder pads need to be re-evaluated for younger players on a regular basis because of this continual growth. Most of us do not live in a major metropolitan area with a large string supply store to experiment with a variety of gear for our violins. In lieu of this, stay up to date and informed of the great variety of chin rests and shoulder pads by subscribing to string catalogs. Here are links to the major string catalogs:
Johnson String Instrument: http://www.johnsonstring.com/
Southwest Strings: http://www.swstrings.com/
Young Musicians: http://www.young-musicians.com/