What are the causes of recurrent mouth ulcers in toddlers?

Viral illnesses: Most mouth sores are due to viral infection, such as "hand foot and mouth" disease, or a herpetic infection (herpes i, not the std). Other viruses can cause an isolated ulcer on the tongue or in the gums. Rarely ulcers can be a sign of an inflammatory disorder but there are usually a number of other symptoms going on. Sometimes the immune system with just cause a random ulcer to appear.

PFAPA: Periodic fever, aphthous stomatitis, pharyngitis and adenitis is a symptom complex of unknown cause that can cause recurrent fevers and sores in the mouth. The fevers are periodical usually every three to four weeks in younger children. Over time it usually abates. Treatment with steroids or Colchicine may be helpful.