I was going to suggest what ngdawg suggested. Prop up the mattress by her head. In fact, with my son and his allergies, we keep his mattress propped all the time. I used a pillow at first, but that squished. They actually sell a wedge, it looks like a small piece of a mattress. That is what we use now, in his toddler bed. It looks like this:
Crib Wedge