lololola, i think you're just freaking out because it seems like a lot of responsibility. Let's face it though, you've already COPED with her for 2 years before this was diagnosed. She was showing symptoms - thirst, perhaps woozy? - and you coped.
You can do this, chick, you're just worried because it's a condition with a name and known risks. Don't be! Now you're better informed, better equipped and have the support of the parents, you can ask advice from them, doctors, other people on here with diabetic children.
Take a while, chill out about it and don't see it as extra responsibility. You are equally responsible for all the children's physical well being. This child has a condition that means you have to check a certain aspect of that regularly - let it serve as reassurance rather than extra stress.
As an anecdote - a friend of mine had diabetes all her life and sometimes slipped into a hypo at school. It's quite simple to deal with, she'd get herself some lucozade or a mars bar and within a little while would feel much better. Now you know the signs, you can check and pretty soon it'll be routine. You can make sure she's less likely to have swings in her blood sugar by planning meals with that in mind. Don't worry. You can do this. You know you can, but I think you're just a bit shocked by it all and worrying that you're going to do something wrong. Ask for support, but I wouldn't be tempted to throw your hands up and give this child notice, it's really not fair on her or her parents, it's discrimination and at the end of the day, once you are more used to the routines involved with her care I'm sure you'll settle into a more relaxed state anyway and will wonder why you panicked at first.
Good luck!