Nicole, I'm so sorry you've been definitely diagnosed!
Is there a local chapter of the National MS Society in your area? You can check in the phone book (under something like "social service agencies") or go to their web site: www.nmss.org and see. They have lots of pamphlets and booklets you can use to help explain this to your children..and to others in your family. I haven't looked at these publications in a long time, but they used to be really clear and simple, and there was even some books like coloring books for kids.
MS is hard enough to explain to adults, much less kids, as it is so variable in each person. And, unlike some illnesses, it's not a matter of: "take these meds and you'll be fine in 6 months". The picture is so much more hopeful now, though, with all the meds available. After you've seen the videos and talked with your doctor, don't hesitate to come back and ask about the meds. I think there are folks on all of them here, and we can tell you about our experiences with whichever one you're considering.
Sounds like your kids are young. I don't have kids, but have counselled folks in similar situations. Best advice I can give is this: you know your child better than anyone else. And you know how much information your child is able to receive and process at any one time. She sounds young. Young children *most of all* need assurances that they're loved, that mommies illnesses aren't their fault, that they haven't done anything to make mommy sick (and even if they're bad, sometimes, that won't make mommy sick!), and that mommy will do everything she can to make sure she will be around for a long long time to take care of them.
They don't need details about what MS will or might do, what you need to do to in terms of shots, or medicines (I was talking to one person who was telling me her child was scared every time she injected her Betaseron. I asked, "...so why are you doing it in front of the child, anyway?? She doesn't need to be in the room while you do that!") What they NEED is reassurances about your love, and your commitment to take care of them, no matter what.
And then, over time, as they get older, and you get more confident, you can offer more information. Or be watchful for their questions, and be ready to answer them.
Sure, there will be times when they'll be upset because they want you to do stuff and you won't be able to. They'll adjust..as will you. And you'll figure out how to be able to do the IMPORTANT stuff, and let other stuff go.
In the meantime, you need to take care of yourself. Do something nice for yourself this afternoon, if it's only a cup of good tea and a few chapters of your favorite book. You'll get past this, in time, and be able to move forward. When you can do that, then it'll be time enough to talk with your kids.