Here's what I found:
They can pose a suffocation hazard before the baby is good and mobile. Even newborns can wiggle around a crib and get trapped under a bumper pad. So if you use them at this stage, make sure they are well tied DOWN, not to just to the sides, of the crib. Bumper pads should have ties on the top and the bottom edges for this purpose.
Once a baby is able to roll over and move around the crib fairly easily, bumpers can be needed to prevent arms and legs from getting stuck in the bars. Some kids have problems, some don't. You can use mesh bumpers to prevent this or regular ones. A crib tent also prevents it.
Once a baby can pull up to standing, the bumpers MUST go. They can be used as a stepping stool, increasing the chances that the baby will get high enough to tumble out of the crib. Mesh ones that cover the entire side and crib tents don't have this problem.
Hope that helps!
at 4:19 PM on Apr. 8, 2009