For most children and teenagers, it is best to be honest and talk to them as much as possible about what is going on. Depending on the age of the child, use language they will understand. Check they have understood by asking them to tell you what they think is happening in their own words.
Some children and teenagers behave differently, or become quiet and withdrawn when someone they know is affected by cancer. It can help to let their school know what is happening so they can get any support they need from staff at the school.