It's not easy to talk to a child about a diagnosis of cancer. The amount of information that you need to give them will depend on their age and emotional development. The most important thing is to be open and honest.
Give your child as much information as you, and they, are comfortable with, and that they can understand. It is best not to withhold information to try to protect them from the truth, as this can cause even more anxiety than if the truth had been told from the start.
It is important to communicate as openly as possible with them and, wherever possible, include them in discussions about their care and their return to normal activities, such as school.