Hi,
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My DS is 6.5 months old and just starting to crawl/get mobile now, so I don't have a ton of advice.
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Have you thought about dropping the frame down on the co-sleeper? That moves the mattress close to the ground and makes it like a pac n play. It is totally safe if the baby is sitting up. If the co-sleeper is in your room now, it would still fit that way.
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We recently dropped the frame on his arm's reach co-sleeper, moved the co-sleeper into his nursery, and installed this bed rail on my side of our bed:
http://www.amazon.com/Regalo-Hide-Away-Bed-Rail/dp/B000CSEE48/ref=pd_sbs_ba_9
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The bed rail is fantastic. We have a queen sized bed that is off the floor with a box spring and bed frame. The rail is very, very secure. I was quite concerned about safety, but once we installed it, I feel totally comfortable. (Of course I still would never leave DS alone in the bed, but when we're in there, it's fine). There is no gap. The corner it makes is as flush as any crib corner would be. It is sturdy enough that I can lean against it.
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So DS stays between me and the bed rail. I have to sit up at night to nurse from the other side, but it works.
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Other ideas -- could you put a small mattress on the floor with a gate/pen around it? Or you could sleep part of the night in DS's room.
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By the way, our DS was not sitting up at all... and then within a matter of 2 or 3 days, he started sitting up, pulling up, and scooting/crawlingÂ
 I was shocked when I gave him a bath the other night and he pulled up and would have tumbled head first out of his little plastic tub if I hadn't been there, eek! Just a warning that it can all happen very, very quickly!
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Good luck!!