There is no discharge from the nipple, no dimpling of the skin in the area, and no evidence of infection. Once someone feels it, however, the family tends to follow it more closely, checking it frequently to see if it changes at all. This can cause it to become sore because of the constant manipulation. I do not think you need a second opinion at this time. By having her examined, the doctor gets a good history, checks to see if there are any other signs of puberty, any evidence of secondary sexual characteristics developing, or other possible causes for the lump.
Reassurance is appropriate along with just observation over time.
If the area seems to be infected, seems to be getting larger, or discharge is noted from that nipple, then I would have her reevaluated.