Without more details, my guess is that it is a lymph node. Young children often have small, firm nodes that are the size of a pea following infections. During the infection, the lymph node "swells" and can become tender and boggy. As the infection resolves, it shrinks and becomes "shoddy". These shoddy nodes can last for weeks up to a few months. They are usually located in the groin area, the sides of the neck next to the throat or behind the ears, or under the chin. As with all masses, your child's doctor should examine it to assure that it is benign. He will ask you about recent health problems, any pain, redness or swelling, any injuries, or any problems eating or with nausea.
HFHS,RN-AM