Lindsey, stomach pain is not considered to be associated with MC because the digestive tract is smooth muscle. There seems to be some question as to whether over time the sphincter muscles in the GI tract can be affected. so it's not impossible.
Children in general often get stomach aches. Sometimes a small amount of magnesium helps and that will also help with the myotonia. Magnesium citrate is a good form - you can get powder and mix a little in juice.