My two year old just started sleeping in a twin bed a couple weeks ago. We put it on the floor so he wouldn't get hurt falling out of it.
I started with him taking his nap in it. It took some getting used to for him, because he'd always slept in his playpen and he was used to having the walls. So, when he first started sleeping on the mattress, he'd roll around and fall out and he asked to sleep in his playpen at first. I let him go back to the playpen a couple times.
I also layed down with him in the twin bed for the first 4 to 6 nights, I think. I'd wait until he fell asleep and then I'd get up... that is if I didn't fall asleep too! I left the door open so he could come to me if he needed me. For the first week, it seemed he woke up a lot and I'd have to go in there and lay down with him a lot.
Eventually I really didn't want to lay down with him anymore... he goes to bed early! So, I quit. It was a little scary for him at first, because he told me he was scared. When he'd say that, I'd ask, "you're scared?" He'd say yes and I'd tell him, "don't be scared... mommy's here, daddy's here, sissie's here and brother's here for you."
That seemed to make him feel o.k. and he'd go to sleep. I don't lay down with him anymore, but he still seems to do a nightly ritual of telling me he's scared and me having to reassure him.
He turned 2 at the end of November.
I think it's a big transition, so maybe you can ease her into it when you think she's ready.