5 y/o with 7 day fever low grade 100.2 to 101.8 comes & goes. Happy, eating, playful. Now says she has leg pain. I asked for CBC today. Leukemia?

Unlikely, but... Getting a CBC is a good way to start. It is encouraging that she seems to be doing well clinically. Any abnormality on her smear (blasts) or abnormal numbers should prompt a more thorough evaluation.