We use a mesh rail on our bed and I personally find it's really helpful. I've never been worried about her falling in the crack, though you could easily stuff it with something. If anything, she rolls into me as a result of the divot my body weight creates in the bed. We have a full bed, though, so I'm more worried about her falling out than if we had a king or something.
We're transitioning away from co-sleeping though. She roots into my arm/boob all night whether she's hungry or not and I'm tired of having two sleep positions (flat on back with arm around her and on my side with arm straight out to side above her head). The past few nights I've been actually getting up to put her back in her bassinet after feedings, which is only acceptable because she's down to one or two night feeds. Two nights ago she only ate once, which was glorious, so I got the bed to myself and only had to get up once. Last night I was up twice, which was worse. But dream feeding, which was so awesome for us at the beginning, now means that she uses me as a pacifier almost all night. She's old enough to sleep without sucking on me all the time, so I think we get better sleep when she's in the bassinet. I don't know when we'll transition her to the crib in the nursery. I agree with everyone else that I wouldn't be comfortable with her that far away. We'll see what happens when she outgrows the bassinet.
(FWIW, Boots, our bassinet has fabric sides (no mesh at all) and Piper unfortunately sleeps with her face pressed fully against the side. We worried about it for a long time, but since she rolls into that position and she breathes fine while in it, we sort of stopped worrying as much. So I wouldn't worry about the co-sleeper too much, but that's just me)
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