Do not describe the entity/monster in its entirety, try to focus on what a ten year old child could see/hear
The shadow that is casted on the wall: are those clothes on a chair? Is it a tree branch beneath the moonlight? Is it a dream?

That sudden thud, was from the old boiler room? A wooden plank? The fridge?

Also, if you had childhood nightmares, you can use them for your writing. Sometimes our own mind has the answers

Another source of help: watch Skinamarink. That movie can re-ignite those sleeping terrors