Children will pick up non-verbal signs that something is wrong and often they will imagine the worst. One of their worst fears is usually that of abandonment. For this reason, they need to be reassured that even though a parent is sick, there will always be someone available to take care of them.
Explain to them in very simple language that mommy or daddy has a disease called cancer. If you avoid the word, you are giving them the message that it is something too horrible to even talk about. Reassure them that the doctors are taking care of things and have medicines for treating the disease. If you know of expected side effects, prepare them in advance. It's important to be honest with them but to also keep it short and simple so as not to overwhelm them.
Invite them to come to you if they have any questions or worries. Although they need to feel that you are in control of the situation, it can be helpful to share some of your feelings, so they can see that it's safe to talk about their feelings. At some point, you may want to mention that nothing they did caused the illness and reassure them that the cancer is not contagious.
One of the questions children may ask is, are you going to die? With CLL a possible answer is, hopefully not for many years--after they are grown. It's also important to explain that people can die from leukemia. If children discover that we lied to them, they will have trouble trusting us again, and this will only create additional fears and insecurities for them.