B complex deficiency in children can cause serious health problems such as anemia, degeneration of nerve fibers, diarrhea, irreversible neurological damage as well as mental instability in children. Children deficient in B complex may be at risk of developing abdominal pain, fatigue as well as nausea. Some common symptoms of B complex deficiency include abdominal pain, abdominal swelling, anemia, diarrhea, bladder pain, constipation, fatigue, tiredness, fever, vomiting, weight loss among others. B complex supplementation can help the child overcome this deficiency.