How do i tell kids grandma has blood and lymph cancer? what do they need to know?

It depends: Dependis on which type of blood cancer she has the prognosis and treatment (and side effects) would be very different. Certain lymphoma's can have a very slow course and in an older person not require any treatment and may only be worth mentioning as a part of grandmothers health. If she has a more aggressive lymphoma you may want to prepare them for chemo effects (hair loss, fatigue, etc).

It is not infectious: The kids need to be reassured that they will not catch the disease from grandma. Other than that you may communicate the diagnosis according to the level of their understanding. It is a cancer, but you may wish to avoid that term and state that she has a disorder of the lymph glands.

I'm not sure what: "blood and lymph" cancer means, chronic lymphocytic leukemia? A lot depends on your kids age, and how close they are with grandma, but this is delicate in respecting her privacy, and your desire to educate or prepare your kids, for what, a discussion of mortality and death and dying or valiant battles with cancer? A mine field without full knowledge and skill, and it varies with prognosis.