The explaining of cancer to a child is not a necessary step to tell the children in detail.
Once your child has been diagnosed with cancer the next difficult aspect is explaining to him that he has this disease.
You may want to shield your child from the knowledge but you do him no good because he is going to sense that something is wrong with him. And since he will be making frequent trips to the hospital and seeing a doctor he will know that he may be ill.
He may also be intuitive enough to feel that you are worried or afraid about something. Some children may feel that they are being punished for some action of theirs.
Someone may let slip that they have cancer and then your child will trust you less for not having given him the information. So it’s best to be upfront and honest and tell him about it otherwise he may get the wrong idea.