I converted Charli's (DD#1) cot to a junior bed with one side rail and the other side of the bed against the wall when she was 18 months. We had no problems with her falling out, but the first few nights we did check on her alot!!! When Charli was about 20 months we bought a single bed, however, it is slightly lower than the average single bed so that if she did fall out she wouldn't have far to fall. I also put her fold out couch next to the side of the bed in case she fell out.

I was a bit worried about how she would go in the big bed as she wriggles alot during the night, but she has not had any problems. I just make sure that she wears warm pj's so that when she does wriggle out of the blankets she is not cold.