I agree with Dr. MattKW. The only way to know for certain is to cut it out and examine it under a microscope (biopsy). Keratosis is the thickening of keratin similar to a callous but abnormal in the mouth. It can be a precursor to Squamous Cell Carcinoma (SCC) which is the most common cancer in the mouth.
In order to obtain one on a 5 year old, we need to anesthetize the area. I'm not sure we need to sedate your child. All involves risks including trauma both physical and emotional. We need to weigh the risks vs benefits.
The old school assessment of oral disease (pathology) is when you hear hoof beats, think horses not zebras. It is very rare to unheard of for a 5 year old to have SCC. According to various sources, a possible lesion similar to the one in the image has a ~97% chance of resolving with no mitigating factors such as irritation, trauma, medications & supplements, or special diet/beverages.
It's is hard to determine from the image but another possibility is the eruption of the adult incisor. I do have a 5 year old at home and if this was in her mouth, I would continue to watch it and monitor for possible mitigating factors.