Oral ulcers in children that are recurring are usually caused by repetitive injury – similar to brushing one’s teeth or the use of braces, vitamin C deficiency, vitamin B12 deficiency, or a zinc deficiency. There are possibilities that the herpes virus could also have something to do with the repetitive ulcers but since you mentioned that these are aphthous ulcers, this can be eliminated as a cause as well as other pathogen induced ulcers.
Aphthous ulcers are lesions along the mucus membrane of the mouth that cause canker sores and inflammation of the area. The reason that an inflammation could take place is possibly because of a deficiency in the intake of certain minerals and vitamins as mentioned earlier but the role of toothpaste and another disease should not be ruled out and must be confirmed by a doctor. SLS or sodium lauryl sulfate is a substance that is found in some brands of toothpaste and this sometimes causes an allergic reaction in the mouth. This allergic reaction can further manifest itself as a canker sores and ulcers so the first step would be to check the ingredients of the toothpaste your child is using for SLS and change to another toothpaste. If your child is wearing braces or if her toothbrush bristles turn flat every month then there is a chance that these two reasons have something to with inciting injury. Braces tend to brush against the skin of the mouth and it is usually a matter of time before an ulcer occurs. The toothbrush bristles turning flat indicate that she is probably brushing too hard and occasional slips are causing injury and ulcers.
The treatment that is advised is to administer some vitamin B12 every evening, sublingually. This can help a great deal in alleviating Aphthous ulcers from the mouth, even if the cause is not a vitamin B12 deficiency. Since pain is also a big concern, whenever these ulcers occur, try to get your daughter to ingest cold foods to reduce the inflammation in the area. Milk and yoghurt are also advised since ulcers are breaks in the skin and this is a chance for opportunistic bacteria to take hold. Milk and yoghurt are rich in a bacterium called lactobacillus. These bacteria are very human friendly and their presence stops the attack of other harmful bacteria and viruses. You should also check with your doctor about the possibility of gluten intolerance or Celiac disease.