I agree with what the other's have said. Paris had a safety sleep and once she learnt how to get out of it, we even resorted to pinning the velcro closed.. she still learnt how to get out of that!
With ayja we never used it as she hated it, she has always been an active sleeper, we go in sometimes to find her sleeping with both legs out the bars, the only thing holding her there is the bar between her legs! (ouch) when we go to bed we move her and tuck her back in.. when we had the side off her cot - (before the mischief saga that resulted in it getting put back on for a while) we would find her asleep on the matress on her floor that we put so she wouldn't hurt herself falling out of bed, other times she'd look as if she were praying, sleeping on her knees with her tummy on her bed.. other times she'd have rolled right off and obviously off the matress and be curled up on the floor!
Now obviously - she's an exception in that she's a more than active sleeper and also sleep-walks, though i must say that it proves that if they want to do it, they're gonna. The thing i realised with the safety sleep is yes - it's meant to keep them in one position - however, just think, if she gets into an unsafe position in that safety sleep then can't get out it's not really that good.
In the end, unless she is seriously threashing about like a maniac, she won't hit her head against the bars of her cot that hard, and if she does get a bit active and bang a bit harder, she'll wake up and cry and you'll know about it.