Lymph nodes are present abundantly in the body and mostly in the neck. They are part of the immune system, and the body fights infection in the nodes most commonly. Small scratch in the scalp or mild cut or infections in the ear lobe may be responsible. Even a throat or tooth problem may be the reason. Usually a course of antibiotics takes care of this. Visit your Paediatrician for the same. If it does not reduce in size , then other things like TB and worse things need to be considered and a fnac test or biopsy needs to be done.