My firstborn transitioned into her 'big girl bed' at 17 months of age. As her toddler bed only had the head & foot area side rails, we were a little worried about her rolling out and onto the floor. Pillows to soften any potential blow caused a problem when she tried to get out of bed in her sleepy-head state because they would trip her up…so I grabbed the changing pad (that we only used for the first few months anyhow!) off the top of the dresser and tossed it on the floor next to the bed. Now we have a firm, cushioned surface for her to plop onto if she rolls out, but it's firm enough to easily walk on without tripping.
I remember worrying about my roly-poly kid hitting the floor when he first moved into a regular bed! But it was a low bed and his room was carpeted, so I figured he'd be fine. He did indeed fall out a couple of times, but he barely woke up. The last time, in fact, we heard a loud thump, we came running, and he had already climbed back into bed and was fast asleep.