Does a dent in the chest of my child mean that someone hit him really hard there?

Not if always there. Pectus excavatum is the most common congenital deformity of the anterior chest wall, in which several ribs and the sternum grow abnormally. It produces a caved-in or sunken appearance of the chest and can either be present at birth or not develop until puberty. The defect is usually cosmetic, but may cause cardiac or respiratory impairment if severe enough.
No. The dent or chest deformity is a congenital pectus excavatum or funnel chest. If symptomatic or causes psychological issues may be repaired using a nuss procedure. Information is readily available on the web and youtube.
No. This probably represents a condition known as pectus excavatum.
Generally not. I would have your pediatrician evaluate the chest to give you recommendations on how to proceed.