Canker sores (also known as mouth ulcers) are common in children. These are usually seen at the base of the gums, under the tongue or inside the cheeks or lips. These canker sores can cause pain to your child. Every day routines like eating food, drinking, brushing teeth all can be uncomfortable. Depending on their location in the mouth, even speaking can be painful. The ulcers are typically round red spots or white spots with a red edge or border.