When my daughter was about 18 months we decided to move her into a "big girl bed". She was starting to climb out of her cotbed and I was worried she would hurt herself. So with a tear in my eye at this toddler milestone, hubby dismantled the cotbed and turned it into a toddler bed.
I was worried that she would roll out of bed in her sleep, so we bought the Summer Infant fold down bed rail (single) for £26.99 from Babies R Us. I believe this is a typical retail price and it is available in most baby stores and occasionally Asda's Baby Event (although they favour the Lindam version usually).
The rail also comes in blue, however we bought the pink one.
Assembling it is very fiddly - it is a never ending connection of metal tubes which you need to be a contortionist to bend back into the fabric. It reminds me a bit of how you adjust shower curtain poles haha. The whole process took under ten minutes, but I have to say the instructions are poor. The rail once assembled has two poles that stick out (as you can see in the picture). These go under the mattress to secure the rail. The rail is meant to fit all junior / toddler /cotbeds as long as they are of typical design. I found that the rail fitted my daughter's bed just fine.
Once assembled there is a button either side of the rail. You press both at the same time whilst pushing down on the rail to fold the rail down. This takes a bit of practice and is a bit of a nuisance to be honest. The rail then folds down so you can make the bed, lift the child out etc. When in place, the rail is a soft mesh material that is supposed to prevent the child falling out of bed.
We had the rail on our daughter's bed for less than a week because on the last night it was up, she stood up in bed crying, leaned on it and promptly fell over the top of it. Thankfully she landed on her soft carpet and was not at all hurt, but this in my opinion was far far worse than if she had rolled out of bed. We removed the rail at once and she has never rolled out of bed - so it wasn't necessary in the first place really!
I think the idea is fantastic, and I like that is cheap and comes in pink or blue, but in my opinion it could be more of a hazard!