Canker sores are common in children.  They are most common from about 4 to 9 years of age.  Never the less, they can occur in younger and older children.  They are painful and can prevent your child from eating and functioning normally.  There are several causes for canker sores and for the most part they are not contagious.  They generally last a few days and clear up spontaneously.  If they persist or recur frequently, then professional advice is required.

Treatment would include soft diet, keeping the area clean and sometimes even ice or popsicles may alleviate the associated discomfort.