How do I tell my young daughter that her mother has cancer?

Simple/positive. For the young child short simple words with repitition over time will communicate the basics. Young kids face some additional needs that can be addressed over time: they did not do or say or wish anything that made mommy sick. There will always be someone making sure she has meals to eat & places to play & sleep if mommy is away trying to get better. Let her ask when ready but repeat often.
Simply & truthfully. You can tell your young daughter that you have cancer in simple language. She may not yet be able to comprehend the serousness or cimplexity of your illness but can certainly understand that 'mommy is sick" this explanation will work for the treatment related side-effects too. Point out to her how others are helping you. She may want to help too-such as gving a hug to help you fell better.