I'm not sure if this is the case in children, but a strong body odour that is not relieved by washing can be caused by a bacterial imbalance in the digestive system. If these children have ever taken large amounts of Antibiotics
or eat a poor diet the 'good' gut flora is wiped out. This leads to nutrient deficiencies and lots of other problems. This might be their case, if it is try supplementing with probiotics. Hope that helps somewhat.