When we removed the bedrail from my youngest son's twin bed, I felt like I was sleeping with one eye and ear open to make sure he didn't fall out of bed. We've been sleeping a lot better since we started using a hack we came up with while traveling with the kids and staying in hotel rooms.
We roll up a large towel (a blanket might work, too) into a long roll, and then shove it under the bed's fitted sheet to keep it in place at the edge of the bed. This helps to give kids a physical reminder if they roll up against it during the night.
It's not foolproof, but seems to be enough to keep my son from rolling off the side of the bed.
I love how this hack doesn't require packing anything extra in your suitcase.