Why did my child get leukemia?

No one knows. Only in rare circumstances do we actually know the cause. Those times would be if the child was previously treated with chemotherapy or radiation therapy, if there is a known genetic problem like down's syndrome or in a few families where it may be hereditary. Exposures, foods, cellphones, high voltage transmission towers, etc have not been proven to increase the risk of getting leukemia.
Dont know. But chances, include environmental, genetics, etc etc, most of thebtime we havebno idea.