They haven't really decided what exactly causes leukemia yet. Radiation is really the only real established risk factor.
As for treatment, it varies. There are many types of Leukemia, as these require different treatment. Often chemotherapy is used in conjunction with radiotherapy. The radiotherapy is full body and any number of chemo drugs can be used.
You can donate bone marrow, but you have to be an exact match, and the procedure is really painful.
Best wishes to the child.
My daughter, who's also 8, is battling leukemia.