What causes a huge dent in my babies chest?

Pectus. This may be a common condition called pectus excavatum and usually does not cause any harm. It is a deformaty of the sternum.
Pectus. Pectus excavatum is the source of the dent. If it worsens over time as the child grows, it can be repaired at a latter date. The optimal age is 11-15 years.