And even if they do roll off, it’s not much of fall at all!
So be sure to baby proof the room first. Think of it as upgrading from the “cage” that was the cot, to a room (if they are getting a room to themselves). They may take longer to fall asleep at first, or they may fall asleep on the floor for the first few attempts but eventually they WILL fall asleep and they WILL stay in bed. It is newfound independence and a natural development that teaches them their own sleep cues and boundaries.
If you are co-sleeping in the same room then you can always guide them back to their beds.