Se subite eksonos tondro en la nokto, la infanoj ne tiom timos, se vi restos apud ili.

In Esperanto, se does not force the present tense the way if often does in English. You normally use whatever tense matches the meaning.

So here:

• eksonos = will start sounding
• restos = will stay
• timos = will be afraid / will fear

All three actions are in the future, so the future tense is perfectly natural.

That means the structure is more logical from an English learner’s point of view:

• Se ... eksonos ...
• ... ne tiom timos ...
• se ... restos ...

All of them refer to a future situation.

Eksonos is made of two parts:

• soni = to sound
• ek- = to begin suddenly, to start

So eksoni means to start sounding or to sound out suddenly.

Then -os makes it future:

• eksonos = will suddenly begin to sound

With tondro, the idea is something like:

• thunder will suddenly be heard
• there will suddenly be a clap of thunder

The prefix ek- often gives a sense of sudden beginning.

Not strictly necessary, but it adds nuance.

• subite = suddenly
• ek- = the action starts abruptly

So together they reinforce the idea of a sudden onset. That can sound very natural when talking about thunder.

Compare:

• sonos tondro = thunder will sound
• eksonos tondro = thunder will suddenly start sounding / a clap of thunder will suddenly sound

Adding subite makes it even more vivid.

Esperanto word order is fairly flexible. The ending -os shows the verb, and tondro has no object ending, so the meaning stays clear.

Starting with the verb can make the event feel more immediate or dramatic:

• Se subite eksonos tondro... = if suddenly thunder sounds...
• Se tondro subite eksonos... = if thunder suddenly sounds...

Both are possible. The version in your sentence puts more focus on the sudden event itself.

Because this is not talking about a specific, already identified thunder. It is more like a general event:

• tondro = thunder, a thunderclap, thunder as a phenomenon

Esperanto has only one article, la, and it is definite. If you do not need definiteness, you simply leave the article out.

So eksonos tondro is like saying:

• thunder will sound
• a thunderclap will sound

Ne timos would mean will not be afraid or will not fear.

But ne tiom timos means:

• will not be so afraid
• will not fear so much
• will not be afraid that much

So the sentence does not say the children will be completely unafraid. It says their fear will be less strong if you stay with them.

Here tiom works like so / that much, modifying the verb timos.

It can work both ways in practice.

• timi literally matches English to fear
• but very often Esperanto uses timi where English might prefer to be afraid

So:

• La infanoj timos can mean the children will fear
• but in natural English it is often better translated as the children will be afraid

That is a normal difference between the two languages.

Because the sentence contains two conditions:

1. Se subite eksonos tondro en la nokto
   = if thunder suddenly sounds in the night

2. se vi restos apud ili
   = if you stay beside them

The main statement is:

• la infanoj ne tiom timos
  = the children will not be so afraid

So the full idea is:

• in the situation where thunder suddenly happens,
• the children will be less afraid,
• provided that you stay beside them.

This kind of double-se sentence is completely normal.

Because apud is a preposition meaning beside / near, and here it describes a location, not movement toward a place.

So:

• apud ili = beside them / near them

In Esperanto, the -n ending after a preposition is used when you want to show direction in some cases. But here the idea is static: you are staying beside them.

So restos apud ili = will stay beside them.

Resti means to stay, to remain, or to keep being somewhere or in some state.

Here:

• vi restos apud ili = you will stay beside them

English could also say remain beside them, but stay is more natural in most translations.

Both are possible, but they are slightly different in feel.

• nokte = at night, nightly, during the night
• en la nokto = in the night, during the night

The phrase en la nokto sounds a little more like a concrete situation or setting. It can help create the scene: thunder sounding in the night.

So this is mostly a stylistic choice, not a major grammar difference.

It refers back to la infanoj.

So:

• la infanoj = the children
• ili = them

This avoids repeating the noun:

• se vi restos apud la infanoj would also be possible,
• but apud ili is smoother and more natural.

Yes, but it would not mean exactly the same thing.

• apud ili = beside them, physically next to them
• kun ili = with them

In this context, apud ili is more specific and more comforting: it suggests physical closeness, which fits the idea of reassuring children during thunder at night.

So kun ili is possible, but apud ili gives a more vivid image.