We moved our first daughter to a twin bed when she started getting out of her crib when she was around 2 and 1/2. I was also pregnant with #2, so the timing was good. We took the box spring out so she was really only sleeping on the mattress on the wood base (making the whole thing closer to the ground). We got a rail set from Target which withstood some wear and tear but has been long gone. Now she is 5, up on the box spring + matress, and just fine.
Our second daughter had to go from crib to toddler bed because we had #3, and the girls had to start sharing a room (baby got the crib and the room to himself) and there was simply no room for another twin bed in big sis's room. This also works well, although the younger daughter gets into the twin bed with her big sister during the middle of the night.
So, both ways are working for us. I think you should follow your gut, and maybe even ask your child for input. If she feels that she has a say, she may be a great sleeper in her new bed.
Tip: we got our toddler bed from a resale shop, so if things wouldn't have worked out, we were only out $50.
Good luck! J.