We know that the risk of SIDS increases with soft, cushy bedding that babies can bury their faces in and not get enough to breathe. We also know that it is best to put babies down on their backs, but when they are strong enough to roll into a different position, the risk of SIDS is usually decreasing. Also, it drops off substantially at 6 months.
Don't worry about your baby hitting her head on the sides of the crib. Head bumping is usually not a problem. They are built for this as they learn to crawl and walk and stand. Getting a leg caught could be an uncomfortable possibility, though. If you want to use something on the side to prevent this, I would pick something that is the lightest and breathes the best. The cushioning itself is not terribly important. I don't know the specific brands of bumpers well, but I think Safety First makes one.
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