J.C.
My daughter was 3 when she was diagnosed. She never had a bullseye rash or fever. She woke up one morning and could not walk and I knew she didn't fall or injure her knee. The doc checked for juvenile arthritis, Lyme and a few other things... The results came back that she had Lyme disease. She was on antibiotic for 28 days. We had to give her the antibiotic every 8 hours. She is 15 now and has not had any problems since.
Obviously if you see an symptoms such as joint pain notify the doctor. Although I am not a doctor I would prefer she stay on the antibiotic for a longer period of time. I know too may people that needed further doses because 3 wks was not enough.