बच्चे को आग से डर है।

Because को is a postposition, and nouns change into the oblique form before postpositions.

• Dictionary form: बच्चा = child
• Oblique singular: बच्चे
• So: बच्चे को = to the child

This is a very common pattern with masculine nouns ending in -ा:

• लड़का → लड़के को
• कुत्ता → कुत्ते को
• बच्चा → बच्चे को

So बच्चा को is not correct standard Hindi.

Here it is singular.

That can confuse learners, because बच्चे can also be a plural form in other contexts. But in this sentence, को tells you it is the oblique singular of बच्चा.

Compare:

• बच्चा आया। = The child came.
• बच्चे को डर है। = The child is afraid.

If it were plural, you would usually get:

• बच्चों को आग से डर है। = The children are afraid of fire.

So बच्चे को clearly means to the child.

Here को marks the experiencer — the person who feels something.

Hindi often uses this pattern for emotions, sensations, and states:

• मुझे डर है। = I am afraid. / I have fear.
• उसे भूख लगी है। = He/she is hungry.
• बच्चे को आग से डर है। = The child is afraid of fire.

So को is not just to in the simple English sense. It often marks the person affected by a feeling, need, or experience.

With डर or डरना, Hindi usually marks the thing feared with से.

So:

• आग से = from fire / of fire / because of fire

In natural English, we usually say afraid of fire, but Hindi uses से in this construction.

Other examples:

• मुझे कुत्तों से डर है। = I am afraid of dogs.
• वह अँधेरे से डरता है। = He is afraid of the dark.

So in this sentence, आग से means of fire in English, even though से often literally means from, with, or by in other contexts.

Because Hindi often expresses this idea with the noun डर = fear, plus है = is.

Literally, बच्चे को आग से डर है is something like:

To the child, there is fear from fire.

That sounds unnatural in English, but it is normal in Hindi.

This is a very common Hindi pattern:

• experiencer + को
• thing feared + से
• डर
• है

So even though English uses an adjective or verb — is afraid, fears — Hindi often uses a noun-based expression here.

Yes. That is also correct and very common.

• बच्चा आग से डरता है। = The child is afraid of fire. / The child fears fire.
• बच्चे को आग से डर है। = The child is afraid of fire.

The difference is mostly about structure and feel:

• डरता है uses the verb डरना = to fear / to be afraid
• डर है uses the noun डर = fear

Both are natural. The second one can feel a bit more like stating a condition or state.

You may also hear:

• बच्चे ko आग से डर लगता है। = The child feels afraid of fire.

That is another very common everyday way to say it.

Because है agrees with डर, which is singular.

In this kind of sentence, the experiencer marked by को does not control agreement. The main noun here is डर:

• डर = fear, singular
• so: डर है

Even if the experiencer changes, है can stay singular as long as डर is singular:

• मुझे डर है।
• उसे डर है।
• बच्चों को डर है।

If the noun were plural, then the verb would change:

• उन्हें कई डर हैं। = They have many fears.

So the verb is matching डर, not बच्चे.

The most neutral order is:

बच्चे को + आग से + डर है

That is:

1. experiencer
2. source/object of fear
3. noun
4. copula

Hindi word order is fairly flexible, though, because postpositions show the relationships clearly. You might hear variations for emphasis, such as:

• आग से बच्चे को डर है।

But for learners, the original order is a very good default.

Because Hindi does not normally use articles the way English does.

So:

• बच्चा can mean a child or the child
• आग can mean fire or the fire, depending on context

Hindi relies on context much more than English does for this.

That is why बच्चे को आग से डर है can be translated as:

• The child is afraid of fire
• A child is afraid of fire

Usually the context tells you which one is intended.

A simple pronunciation guide is:

bach-che ko aag se dar hai

A bit more carefully:

• बच्चे = bach-che
• को = ko
• आग = aag
• से = se
• डर = dar
• है = hai

A few notes:

• च्च in बच्चे sounds like the chch in watch-chain
• आग has a long aa
• ड in डर is a retroflex d, made with the tongue curled slightly back
• है is often pronounced close to hai or sometimes a bit like heh depending on accent and speed

Usually it means a general fear of fire.

So आग से डर is normally understood as fear of fire as a thing, not necessarily one particular fire right in front of the child.

If the speaker wanted to point to a specific fire, context would usually make that clear. But by itself, this sentence most naturally means:

The child is afraid of fire.

That is, fire is something the child fears in general.