that's Lymph node ....."Lymph nodes are normal structures present in every body. They produce part of the White blood cells called lymphocytes.
During infection, they get bigger because they are working extra hard to deal with infection.
It is nothing to worry about... The most important thing is to deal with the underlying cause of the lymph nodes enlargement..... Kindly see a doctor preferably a Paediatrician for thorough evaluation and management

Never leave children alone as for the swelling please apply ice packs and watch for any reactions.