In July 2012, my husband and I heard the words "your daughter has a brain tumor," and our lives changed forever. You hear about adults being diagnosed all the time, but we didn't even know that children could have cancer.

At the time, Hannah was only 4 years old and getting ready to start school in the Cayman Islands, where we live. But when the doctors said she had anaplastic medulloblastoma, an aggressive type of brain...