I think you must be really, really careful when it comes to this kind of thing. You know your kids best, so tell them what they can handle. I mean, when they're really young you can say "Mommy went away for a while because she's sick" or something to that effect. As they get older you can elaborate on what she's sick from.. and then teach them about it. Let them know that she loves them and she's sorry or whatever you need to say to make it easier for them