DD was 18 months when we moved her into her big bed. She also slept all over the cot so we decided to put a mattress on the floor. Amazingly, she stayed there all night. I dont think she realised that there were no sides and that she could climb out!

After a few nights of the same, we put her in the bed, and put an extra mattress on the ground beside the bed in case she fell out. It only happened once, and all has been good since. We don't, and never have used rails either. Just placed the bed up against the wall on one side, and left the other side open.