I think this depends a lot on the child and how much they understand. I always had a big bed set up in my boys rooms (alongside the cot) and we would sit there for stories before bed and then put him in his cot for sleeping. My first son asked to sleep in a big bed when he was 18 months old. We put some quilts on the floor next to him for padding and he rolled out a few times but was much happier in there.

DS2 went to a big bed when he was 15 months. We asked him if he wanted to sleep in the cot or the big bed and he pointed to the big bed, so he slept in there. He was fine in there and only fell out once. The only thing I can think of is that once they are in a big bed they are 'free to roam'

You need to have a relaxed outlook, or be very stern with them so that they stay put once you get them in bed.In saying that we didn't have any problems until we put them in the same room. DS1 still stays in his bed but DS2 is a bit of a yoyo!

Good luck!