monsters under the bed

We have all had nightmares and those of us with children will be familiar with the screeching or crying child in the middle of the night that claims that monsters are under their bed or in the wardrobe.
So what do you do with a child that insists they are seeing things?
My advice would be not to dismiss it out of hand. Children are very perceptive and some see and sense spirit but also they can pick up on any disturbance within the household and manifest symptoms so that they get attention (not intentionally) of some kind.
For the child that tells you they can see monsters or someone in their room I would be sympathetic and gentle. Ask what the child saw, how it felt, what it said or did and how it made them feel? Where they think it is now and if it's been before or is likely to come back. Do they recognise the person or get a sense of why they have come to see them?
Such questions will reassure the child that you are interested and you will get a measure of what they may be seeing or if it was just a dream.
I would not tell a child to stop being silly or accuse them of making things up. It's important to keep communication channels open with children of all ages and supporting them is a parent's job - even if it is in the middle of the night. A good tip is to empower the child by giving them a technique to send the beasties away. You could have them spraying magic spray into the room to cleanse it, you could give them an angel doll or crystal that will guard them and so on. Protection is about belief of being safe and these mental techniques work.
Children who see spirit are not obvious as the movies depict. They are happy, lively children with friends and interests. This is what draws spirit to them.
As an adult who sees spirit people and energy it can at times be frightening, especially before you understand what you are seeing. With the knowledge and experience I have I can usually work things out quickly but for a child, seeing things that don't make sense can be terrifying. It doesn't mean that they will always see or grow up to be a medium. As we grow into adulthood our responsibilities often take the place of sensitivities we have as children and we become conditioned and numb to our extra senses, then we have to work hard to get them back again!