They hide under there blankets n stare up again n hide n look n hide but crying n fearful face

Your children are probably seeing shadows on the ceiling and they think that they are actually seeing a monster and it is scaring them.

When I was little there was a Weeping Willow outside my bedroom window and at night sometimes I would think I saw a monster in my room until I realized that what I saw was actually a shadow of the tree limbs from the Weeping Willow moving in the wind.

How old are they?

As another asked, how old are they? Do you let them watch horror movies, or anything that is really distressing? Some kids have nightmares when they see bad movies, and it really scares them. As another said, they could be seeing shadows.
Why don't you sleep in the same room with them one night, and see if they are OK with you there? It could be they are just afraid in the room by themselves. It should go away in a while. Good luck, and hope you have a great new year.
PS; Has anything happened to them recently, like their Dad leaving or something?