Most bow sounds that emerge from young learners are less than brilliant, and that's because there is so much to think about and its pretty darn difficult...
The majority of problems come when the bow moves too slowly producing a scratchy sound. I explain to my students that it's like a car with the breaks on, if it's not continuously moving, the car will produce a screechy sound. The bow is like a moving car, it has to keep moving at a continuous pace. The faster the bow, the more brilliant the sound, as long as (and this is important) there is not too much bow pressure! crushing the bow into the string is the other mistake kids can easily make. The string needs to vibrate in order to produce a brilliant sound. If you press to hard the string can't produce a ringing sound. Particularly with chords, a lot of bow is needed, and a fast arm - almost like they are pushing someone away. If the sound is on the other hand too quiet, then they are also not moving the bow fast enough. Lastly, check the bow grip is firm, and the arm is at the right string level. A lazy bow grip means the bows contact with the string will be not as effective, and it won't matter how fast the arm is moved!
Here I write my musings on teaching, or other things that come up in lessons/relate to violin learning. Anything suggested is just that, suggestions and my thoughts and may differ from the opinions of my other music colleagues!