Having a family member with CLL increases one's risk of developing the disease. You need to tell your children for THAT reason alone. Your DH needs to understand this - First-degree relatives (such as children) of CLL patients have double the risk for developing CLL themselves. What's worse, the average age of diagnosis of the second-generation (offspring) is nearly 20 years younger than the parent's age at diagnosis.
Also, I know you said the "prognosis is decades", but do you know the type of genetic mutation your DH has? It can GREATLY affect the overall life expectancy of the CLL patient. Certain mutations have an overall survival of 8 to 10 years, while others have an overall survival of 20+ years.
Additionally, while CLL itself is a "slow" cancer, it CAN have serious complications (like Richter's transformation.) It CAN also "transform" or "convert" into far more severe, serious, and aggressive cancers/diseases, like lymphoblastic lymphoma, hairy cell leukemia, "high grade" T cell lymphomas, acute myeloid leukemia, lung cancer, brain cancer, melanomas of the eye/skin, salivary gland tumors, and Kaposi's sarcoma.
What would you then tell your children if something like that happened?
Your 16-year-old absolutely deserves to know what is going on with their DF - as I guarantee you they'll be able to pick up on the fact that something is wrong with their DF.
When they inevitably find out later on that you and/or your DH kept the diagnosis from them, they WILL feel betrayed and hurt and may not forgive either of you for it. Especially since they THEMSELVES are at a greatly increased risk of eventually developing CLL.