This is a hard one. My son is now 7 and I've only recently started explaining to him what my meds are for, why I take them and what's "wrong" with me; his therapist and I came up with a good starter:

"You know how Grampa has diabetes? And how he has to give himself those shots every day to keep him healthy? And how Mom-Mom has to take medicine for her heart because it doesn't work the way it's supposed to and if she doesn't take them, she could get very sick? Well, Mama has a disease in her brain, one that keeps it from working the way that it's supposed to, and the medicine I take keeps me from getting sick."

And there have been discussions about "mental illness," and how that term is widely used but how, in our family, we like to call it "brain disease," because the brain is an organ just like the heart or the liver or the lungs and all organs can get sick and require medicine to help keep them working properly. I do this because kids tend to repeat EVERYTHING and I would hate for him to tell his school mates that his mama is "mentally ill," then be stigmatized (him, not me) for having a "crazy" mama.