My 4 year old has a tiny pea size lump beneath the skin (not tender, not red and movable) located just off the knee on the inner part of the leg.

Benign mass knee. Non-tender, mobile mass which is not red/inflamed may indicate a benign mass. how fast it grows, how hard is it and whether there was previous trauma/infection would help your doctor make a more informed decision about what it could be.