Parents know their children best and can expect that their children will react in ways that are typical of their personalities. For instance, a child who is very dependent may become even more so in the throws of a new cancer diagnosis. A child who always imagines the worst may do so now as well. A child who plays rough with his toys when upset may get even rougher. Children are often unable to express how they are feeling in words.
Most parents get an idea of what is going on with their kids by watching their behavior.
So, a parent who is observing their children fighting with each other more now can probably assume that this is their way of showing they're upset. Teens may want more information about the disease and treatment, and feel less fearful if they gain more knowledge. Create word pictures to encourage the child to have specific images about the illness. You may even want to rehearse what you want to say in advance. Don't burden the child with your fears and your tears.