In my world, there is a monster that materializes as a childhood nightmare (nothing specific, just this general fear children feel when they are alone in the dark). I wonder what would be a nice way of defeating it? Light (which works for children in reality) would be too obvious and easy.

In this case, I am looking for an offensive measure that would still be somehow symbolic.

Distraction via Good Memories

If a child's fear is what materializes the "monster" then the lack of fear is what will defeat it (duh face palm answer down voted). Working with children in the past I have found that not reacting to what is causing them fear but staying calm and even talking about something off topic to be very helpful.

I was able to keep a child calm after he had fallen and his arm looked like a squiggly Tetris shape. He was crying and holding his arm being like a champion walking towards the building when my coworker saw his arm and started freaking out about calling an ambulance and .... after that the child was lost to his emotions and fear.

Breaking a child's focus on the danger / fear and giving a calm rock to feed off of will help dismantle the monster. (This is also dependent on a strong relationship between the child and the adult)

In terms of story having adults ask children to play, dance, bake a cake, ... anything to get the child to stop thinking on what is causing them fear. Every child who might be a target would have an adult tasked to keep the child's mind off of the monster.

Option 2: If the fear is just to intense then drugs. Just find a drug that prevents user from feeling fear. I heard that cocaine makes you feel god like maybe something similar or a completely made up one.

Rubber mallet.

I have used this method successfully (n=2). Children feel defenseless in the dark. You don't want to give them a shotgun or a Bowie knife. You can give them a rubber mallet. A rubber mallet is not a toy. They are big, hefty tools. Even a kid can deliver a wallop with one should something need walloping, but she is unlikely to hurt herself by accident.

It is an empowering feeling, holding a rubber mallet in your hand, ready to whack something. Even for an adult! Also the rubber is grippy, and the mallet will stay put under a pillow.

Lucid dreaming- through composure and self discipline, the child can be trained to become aware that the nightmare is a dream, and gain control over the dream's narrative. This could allow the child to actually fight back against the monster.

Cats†

Have you ever wondered why cats will sometimes stare, or even vocalize, at seemingly nothing? Suddenly dash across the house like a madman... err, mad-feline. It is because they can see these monsters forming. Or their fading remains. And cats are supernaturally gifted with claws and fangs that can shred the shadows of terror as easily as drapes. They can fit easily under the bed to reach the monsters. And like to curl up next to sleeping children to keep them safe.

Which is why you should never declaw your cat.

†Or dogs if you must

Blanket over head

I can't recall where I read the story (but it was probably an old one) and it's author or title, but there was a sci-fi short story that involved materialization of your fears - a chemical that made you see hallucinations that felt real enough that if the hallucination would kill you, you'd die. When the two protagonists understood the principle, they remembered the secret weapon against each of the monsters they had invented during their childhood - some were vulnerable to a water gun, some to loud noises, some feared mirrors, etc.

But then they encountered a horrible monster whose special power was that no secret weapons work against it... until they remembered what gave them invulnerability back when they were children which was to hide under a blanket - if you're shivering together with your buddy under a blanket with a flashlight, you may be scared, but none of your childhood nightmares are actually going to get you.

A not serious answer, but if you want a more comic tone you could find it interesting...

You could try to trick the monster. Since it thrives in the dark, it hasn't good sight, so you could disguise a brave full-grown man (maybe a short one :) ) as a kid and let him in the child room with the lights turned off.

When the monster tries to manifest itself to frighten the child, the man greets it with a laugh and makes fun of it saying it isn't that frightening...

The poor monster, discouraged, flees in tears and won't be heard of anymore!