L.B.
Any kid can widdle themselves through a good size opening...the question becomes (hopefully their head doesn't fit through it) but can they get their arms/legs back through it on their own? My daughter used to get so frustrated with her crib she'd stick her arms and legs through it and then couldn't get them back out...
Another option that you could consider is getting the bed protectors (like the kinds that fit under the matress and prevent them from rolling out of the bed)...if you put it on both sides then it should protect her for the length. As for the short width ends...I wouldn't worry about them since you'll probably have a pillow there.