Depends on the age
Depending on the age you let them know mommy is sick.
You may consider who and where you are going to tell them, as you will need a great deal of support as well, be honest and candid, use language that they can understand, let them know that it is not their fault, nothing they have done. Assure them mommy and her docs are doing everything to make you better, let them help you. They need security.