For context, this would be from the POV of children who are trapped in a high-rise office building during a monster attack in the city. They wouldn't have any injuries from the event; they would just be unable to move.
I once met a man who had been a similar experience; he was a firefighter in the 9/11 attacks. He had been up in one of the towers while it collapsed, and was buried for over three days. Here's what I remember him describing it as:
It's dark. Pitch black. You can't see anything, nothing can see you. There are no sounds, only silence; and if there are any, they're unrecognisable. The only way you know where you are is to feel around. You'll feel the rubble above you, below you, everywhere. It takes a while to realize what happens; chances are you were unconscious from the impact. And despite the darkness, you never can sleep.
You feel hopeless. He had fired his gun up through the rubble, to try to get people to find him. You try everything you can, except scream. You have limited air; you waste it with screams. You can't move, either. You can't do much, and quickly your only choice is to wait. To hope. To pray.
I would give you his name, but I don't quite remember it. Although I'm sure plenty of similar stories exist.
Provided that you cannot come up with what your character could feel and think, you have several options to get some fresh ideas.
Check records from actual events
- Check the records of earthquake survivors.
- Check the records of newscasts from rescue operations of people being trapped in caves, or under debris.
Read how other authors have dealt with the scene
These could include also watching movies in which the characters get trapped under the snow, or buried alive in a coffin by some sadistic villain. The movie 127 hours goes to the extremes of showing the feeling of being trapped and unable to move.
Create a roleplay out of it
It may sound silly, but try wrapping yourself in a blanket under a pile of pillow to the point that you cannot move and need help to get out. Imagine that the pillows are the debris. Make mental notes of your feelings and of your thoughts. You can play MP3s with sound effects from police sirens or monster cries to aid your sensory experience.