7yr old found a pea sized lump few inches above nipple, next to armpit. Not sick, doesn't hurt, not red etc. What could this be? Thanks

Likely lymph node. We have hundreds of pea sized lymph nodes scattered throughout the body. They are more often noticed in the neck where they are not buried in tissue & may enlarge with local skin irritation/throat infections. A close exam of your kid may find other examples in the groin or neck. They are pea sized at rest. Your doc can confirm this. I would discuss it at you next regular encounter.