yakisoba no morituke ga kirei da to, kodomotati ha sugu tabetagarimasu.

It means the plating/presentation of the yakisoba.

• 焼きそば = yakisoba
• 盛り付け = arrangement on the plate, plating, presentation

So this is not talking about the taste yet. It is talking about how the yakisoba looks when served.

Here, の links two nouns.

So:

• 焼きそば = yakisoba
• 盛り付け = plating/presentation
• 焼きそばの盛り付け = the plating of yakisoba / yakisoba presentation

This is a very common noun-linking use of の. You can think of it as something like of or as making one noun describe another.

が marks 盛り付け as the subject of きれいだ.

In other words, the sentence is saying:

• 焼きそばの盛り付けがきれいだ = the yakisoba presentation is pretty

Here, what is pretty?
The answer is 盛り付け, so it gets が.

Using は instead would change the nuance and make it more of a topic/contrast. が is the most natural choice here for identifying what has the quality きれい.

Because きれい is a na-adjective.

In plain form, na-adjectives use だ when they come before things like と in this kind of structure:

• きれいだ = is pretty / is neat

So:

• きれいだと = if it is pretty / when it is pretty

In casual speech, people sometimes leave off だ in some places, but in standard grammar here, だ is the correct form.

Here, と is being used as a conditional, meaning if or when.

So:

• きれいだと = if it is pretty / when it is attractively presented

This と often expresses a natural result, repeated result, or predictable reaction.

So the sentence has the feeling of:

• When the yakisoba is plated nicely, the children immediately want to eat it.

It sounds like a general pattern, not just one random event.

子供たち means children.

• 子供 = child / children
• たち = a pluralizing or group-marking suffix

So 子供たち emphasizes that we are talking about the children as a group.

Japanese does not always have to mark plural, but たち is often used when the speaker wants to make the group explicit.

The は marks 子供たち as the topic of the main clause.

So the structure is roughly:

• As for the children, they immediately want to eat it when the yakisoba presentation is pretty.

The first part, 焼きそばの盛り付けがきれいだと, sets up the condition.
Then 子供たちは introduces the main statement about the children.

This is a very important point.

食べたい means want to eat, but it is usually used for your own desire or for desire you can state directly.

For a third person like the children, Japanese often uses 〜たがる to describe an observable desire:

• 食べたがる = to show signs of wanting to eat
• 食べたがります = polite non-past form

So 子供たちはすぐ食べたがります means something like:

• the children immediately want to eat it
• the children immediately start acting like they want to eat it

This phrasing fits well because the speaker is describing the children’s visible reaction.

It suggests a habitual or repeated reaction.

Because it is in the non-past polite form and combined with the conditional と, it sounds like:

• Whenever it is plated nicely, the children tend to want to eat it right away.

If the speaker wanted to describe one specific moment happening right now, 食べたがっています might be more likely in some contexts.

So 食べたがります here feels like a general tendency.

すぐ means immediately, right away, or quickly.

Here it tells you how fast the children react:

• 子供たちはすぐ食べたがります = the children immediately want to eat it

So the nice presentation has an instant effect.

In this sentence, きれい means pretty, attractive, or neatly presented, not simply clean.

きれい can mean both beautiful/neat and clean, so context matters.

Because the topic is 盛り付け (plating/presentation), the natural meaning is:

• visually appealing
• nicely arranged
• beautifully presented

It sounds more like a general tendency or repeated pattern.

That comes from the combination of:

• 〜だと = if/when
• 食べたがります = tend to show desire to eat

So the sentence means something like:

• When yakisoba is presented nicely, the children immediately want to eat it.

It is less like a one-time narrative sentence and more like an observation about what usually happens.