Children and cancer should, in a perfect world, be mutually exclusive; They should have no business with each other. Unfortunately, this is not the case. One of the hardest things anyone, parent, uncle, friend or doctor, can do is to tell a child they have cancer.
A child should be told they have cancer before the treatments start. This will help them deal with the discomfort and exhaustion often seen as side effects of these treatments. How, and when to tell them is definitely a matter for the parent or legal guardian to decide. But what is important is to let them know that they are sick.
Exactly what to tell a child depends on their age. Keep in mind their level of thinking, and always keep things simple. Children will often blame themselves for their misfortune. It is important to tell them that having cancer wasn't their fault, and they will get through it one way or another.