We used a Munchkin one (at Target, one of the few reasonably priced that works on a king sized bed) with DS from about 6 months through 10 months. If it fits properly, there really shouldn't be any gap. I also liked the height of it; it is fairly tall. I have since given it to a friend to use with her babe, he is 11 months now, and they enjoy it.
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DS still co-sleeps (from about 4 or 5 a.m. until 7-9 a.m) and we no longer use the bedrail. He hasn't tried to get off the bed, usually walks to edge and asks to get down. Or, rarely, goes down feet first, the way we have taught him to get down off of chairs and sofas.