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It is possible that your grandson, might have developed a hematoma in his cheek as a result of injury.
Depending on the size of the lesion, the healing will require upto 7 to 10 days.
To quicken the healing and also to prevent development of any scar, it would be helpful to seek your doctors help. Prescription of pain killers and anti-inflammatory medications might help to speed up the healing. In case of large hematoma, drainage of the collected blood by a small incision would also be necessary.