Just posted this in another thread - so we went straight to normal bed (well is bunk bed but is normal single size but bottom bunk is actually only about 30cm or so off floor) - no rails (reasoning was rails don't go whole length of bed anyway, and at some point would have to get used to without rails)

Have just transitioned DD into a bed at 21 months in preparation for DS arriving in a couple of months. We just got bunk beds from IKEA with a trundle bed underneath, for the first month just pulled out the trundle in case she fell out (the bunk is only about 30cm from floor anyway - we decided against rails) - found her on the trundle asleep a few times, and now don't even pull that out - sometimes her legs are dangling out, but she has fallen out of a slightly higher bed while on holiday and didn't even wake up. She loves her new bed and has gone to bed better than did in cot - we lay down next to her and read three stories then kiss her and teddies and say good night leave room and thats it. We haven't found her wandering about overnight, but DH did discover her in our room once when she was meant to be having a day nap - I use the monitor still so I can hear her if she wakes up, and normally I get up if it sounds like she gets out of bed (often I hear her 'reading' to herself in the morning). I ask her in the morning if she has slept well, she always says yes and really seems to like it - she is often laying across the bed or upside down etc though, but I just cover her back up with the doona before I go to bed.