I have a 18month old DD and I put her into a big bed on the Anzac weekend for the first time, due to her also moving around heaps and bumping her head and she general didn't like the cot either. I have had no problems, At night time she general will just go to bed when she is ready at about 7.00ish (always been a good sleeper so that helps) and doesn't get out of bed until I go in and get her in the morning. I don't have a rail on the side but I have put a rolled up blanket under the side of the bed so it forces her to stay in the centre or towards the wall side of the bed. She has only fallen out once and that was at the very end of the bed so I now also but a pillow under the end of the mattres and that causes her to also stay in the centre of the bed.
So ye give it a try maybe making the bed up with her and letting them sleep in it for day sleeps and then go to night sleep. I close the door with DD and always have since birth so I don't have any problems with her coming out.
Our Princess Kaylee Louise