Sores in the mouth are common when a new orthodontic appliance, such as a removable plate, is fitted. They appear as a result of friction between the sensitive lining of the mouth and hard or protruding parts of the appliance. Even so, children's mouths are extremely adaptable, and in my experience children rarely complain about this problem.
Usually, even without any special treatment, the sores heal within about a week and don't come back. However, if your child is in more discomfort, there is no need for them to put up with it. In that case, the first step is to see the orthodontist, who can smooth down the problem area on the appliance.
To relieve the symptoms, the sore can be treated with:
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A solution of water and bicarbonate of soda
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Cotton wool dabbed with hydrogen peroxide
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Liquid vitamin A
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Camomile tea
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Various specialised products recommended by the pharmacy
In most cases, once the orthodontist has adjusted the appliance, the sore will heal even without any further treatment.
This answer is for general information. Your own case needs an examination by an orthodontist.