Dr. Green's book says that many people with celiac have had frequent canker sores as a child. Often it is a vivid, early memory that people with celiac have from childhood. My daughter, when she was a toddler, used to call them "cold cuts." We thought it was a tendency she inherited, because her father had them too. They disappeared on a gluten free diet for both my husband and daughter.
Interestingly, according to an article in the journal BMC Gastroenterology by Dr. Farhad Shahram, a researcher at the Tehran University of Medical Sciences in Iran "It has been reported that in five percent of celiac disease patients, aphthous stomatitis [persistent canker sores or open ulcers] may be the sole manifestation of the disease."
If a child or adult has chronic canker sores in his or her mouth, I would recommend a test for celiac disease be done before trying a gluten free diet. Ideally, genetic tests would also be done, which would indicate whether celiac disease is a possibility. Once testing has been completed, it might be useful to try a gluten free diet to see if the canker sores clear up. If they clear up on a gluten free diet, but reappear upon introduction of gluten then gluten intolerance might be a factor.
Canker sores in someone who is already on a gluten free diet might indicate they are being exposed to gluten. Alternatively, sores might indicate vitamin deficiencies.
The mouth is the beginning of the digestive tract so problems in the mouth can sometimes be an indication of more serious problems inside the gastrointestinal tract. If your child has other developmental issues in addition to canker sores, paying attention to this small symptom as a possible sign of celiac or gluten intolerance might shed some light on what is driving the developmental problems.