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One day, you are accosted by a wizened, blind man who refuses to let you pass unless you answer the following riddle.

As a verb, I can create what is later erased from this world,

As a noun, I may steal away children and change them forever.

As an adjective, I will laugh at those who fail to believe me.

I believe we are looking for ...

A homophone! The man cannot see, but he can hear, so they are all the same to him.

The word is therefore...

WRITE / RITE / RIGHT

Explanation:

As a verb, I can create what is later erased from this world

Whatever is written can later be erased - WRITE

As a noun, I may steal away children and change them forever.

Many cultures hold celebrations and special events for children as they mature - RITE

As an adjective, I will laugh at those who fail to believe me.

If you don't believe me, you're wrong because I am - RIGHT

An attempt that falls short:

draft

As a verb, I can create what is later erased from this world

If you draft a document, it will get erased when the final version is adopted.

As a noun, I may steal away children and change them forever.

In case of a war, the draft brings the children of a country under its flags, and the military experience will change them forever.

As an adjective, I will laugh at those who fail to believe me.

I don't know anyone who doesn't believe in draft beer (or draft animals), but if you're in that case people might laugh at you.

Is the answer ...

Imagination


As a verb - "Imagining" other worlds/things/idea/objects; erasable, of course.

As a noun - "Imagination" steals children and inspires them to be better, changing them to be better.

As an adjective - "Imaginative" people might scoff at others when they / their ideas aren't understood by others.

Partial Answer

I have two comlpletly different answers to the riddle. The logic will be very strange in both explanations. I also know both are probably wrong, but I tried.

First Answer is:

Act


Act, as a verb means doing something. From a certain point of view, no act last forever, it will be lost in time at a some point. But sound depressing.


Act as a noun by the definition of a dictionary is the doing of a thing .
Also there are at least two derived nouns which partly fit the riddle. Activity and action.
A certain action or a certain act, or even a certain activity at a certain time can lead to the described situation.
Again the logic is a bit far-fetched here.


Act as an adjective is also not used, but active does exist.
Now here is the most far-fetched thing.... Acive means working, doing, happenning, ect so going by that, if you don't believe what's happening right before your eyes, then you can be laughed at.


A big thank you to @Quark-epoch for pointing out that Act can be used as a noun


A big thank you to @Omega Krypton for pointing out that Act can also refer to a story. It makes Act as a noun fit as an answer better.

Second Answer is:

Life


Life as a verb is Live, and it will sound even more depressing, but it ends at some point in time.


Life as a noun is ever changing. In life anything can happen. Also sounds a bit depressing.


Life as an adjective in the simlest examples is lively and alive.

Again, lively doesn't fit the riddle well.

Alive, however fits to a certain point. If you don't belive you are alive, that can be depressing. But the life is still there.

As I said, the logic behind these answers is very strange and downright depressing, but I have nothing more on me right now. I am still trying for solutions.

My Guess:

Lying

As a verb, I can create what is later erased from this world,

Acts of lying are often found and clarified.

As a noun, I may steal away children and change them forever.

Lying steals their honesty away, and may make them a deceitful person, for life.

As an adjective, I will laugh at those who fail to believe me.

Something lying is not worth trusting, and you'll be laughed at if you believe it! (The last line of the question is a lie...)

Attempt:

Dream

As a verb, I can create what is later erased from this world,

Dreams are very hard to hold onto in most cases, and tend to disappear from memory as soon as one wakes. So the act of dreaming tends to create something temporary.

As a noun, I may steal away children and change them forever.

Maybe a dream can lead to an epiphany. Or, maybe the child has a dream of what they want their future to be, so they do what they consider they must to follow that path and make it happen.

As an adjective, I will laugh at those who fail to believe me.

To be dreamy is to be desirable, but sometimes unreal as well. Perhaps though, the idea of something dreamy is simple imaginative, and if the person fails to believe it, or entertain the thought rather, then perhaps you laugh at finding them to be lacking an imagination, or find them to be too serious.