1. Load your stage. The visible floor that belongs to the stage is now probably at the height Y=0.
2. At frame 0, please move the entire stage downwards until the top of the bed is at the height Y=0. Register the stage at this negative height.
3. Load your female model and let her lay down on top the bed. Her ponytail or twintails will not clip down inside the bed, because the top of the bed is now at the height Y=0.
That's all - simple isn't it?