Children always have mouth ulcers, may be the lack of trace elements, trauma, low immunity and other reasons. 1. Lack of nutrients: children always have mouth ulcers may be the lack of nutrients, if the children usually picky eaters or irregular diet, it will lead to a variety of nutrients in the body, such as vitamin B ₁, vitamin B ₂, vitamin C, zinc and so on. The lack of these nutrients can induce or aggravate mouth ulcers. 2. Trauma: some children will have the bad habit of sucking fingers and gnawing hard objects, which can easily lead to trauma in the oral cavity, and the infection of wounds can easily lead to mouth ulcers, and the most common part of such ulcers is the upper jaw. 3. low immunity: if the child’s immunity is low, it is easy for bacteria and viruses to invade the inside of the mouth, thus leading to the child’s mouth ulcers. If a child has mouth ulcers, it is recommended that the parents take the child to the hospital to clarify the cause of the disease and treat it under the guidance of the doctor.