There hopeful side. You would not wish your children have an abnormal fear of or illness. Children may find out the truth from someone else, or get misleading information from TV or other sources. Children can feel isolated if they are not told. They might feel they are not important enough to be included in a family matter.
Children are good at noticing things but poor at understanding what they mean.
Now Dad has to go into hospital. Then she was ill. Children who know the situation can be a comfort to you. You won't need to feel secretive and isolated in your own family. Children have an amazing ability and capacity to deal with truth.