kodomo no seityou ha hayai.

The usual readings are:

• 子供 → こども
• の → particle の
• 成長 → せいちょう
• は → written は, but pronounced わ when it is the topic particle
• 早い → はやい

So the whole sentence is read:

こどものせいちょうははやい。

の connects two nouns.

Here:

• 子供 = child / children
• 成長 = growth

So 子供の成長 means something like:

• a child’s growth
• children’s growth
• the growth of children

In this sentence, の is showing that the growth belongs to, or is associated with, children.

は marks the topic of the sentence.

Here, the topic is:

子供の成長 = children’s growth / a child’s growth

Then the sentence says something about that topic:

早い = is fast / is quick

So the structure is:

• 子供の成長は = As for children’s growth,
• 早い = it is fast

A natural English translation may be Children grow up quickly or Children’s growth is fast, but in Japanese the grammar is built around the topic.

That is also a very natural sentence.

• 子供の成長は早い
• 子供は成長が早い

Both can mean roughly Children grow quickly.

The difference is mainly in what is being focused on:

• 子供の成長は早い
  focuses on the growth of children as the topic.
• 子供は成長が早い
  focuses on children as the topic, and says that their growth is fast.

So the original sentence is slightly more centered on growth itself.

No. In this sentence, 成長 is a noun.

It means growth or development.

That is why it can be followed by は:

• 成長は早い = growth is fast

There is also a verb form:

• 成長する = to grow / to develop

So you could also say something like:

• 子供は早く成長する。
  Children grow quickly.

That version uses the verb 成長する, while the original sentence uses the noun 成長.

Both 早い and 速い can be read はやい, but they are used differently.

• 早い = early; fast in the sense of time coming sooner
• 速い = fast in speed or physical motion

With growth, Japanese normally uses 早い, because the idea is that growth happens quickly over time, not that something is moving at high speed.

So:

• 成長が早い = growth is fast / develops quickly

Using 速い here would sound unnatural.

It can mean either, depending on context.

Japanese nouns usually do not change form for singular vs. plural the way English nouns do. So 子供 can mean:

• a child
• children

In a general statement like this, English often translates it as children:

• Children grow quickly
• Children’s growth is fast

But a child’s growth is fast is also possible depending on context.

Because this sentence is in plain style:

• 子供の成長は早い。

A more polite version would be:

• 子供の成長は早いです。

Both are correct. The difference is politeness, not basic meaning.

• 早い。 = plain
• 早いです。 = polite

No. You may also see:

• 子供
• 子ども
• sometimes こども

All of these can be used.

A lot of modern writing prefers 子ども because the second part is often written in hiragana. But 子供 is still very common and widely understood.

So in this sentence, 子供の成長は早い is perfectly normal.

Japanese often likes to describe things in a more noun-based way than English does.

English often prefers a verb:

• Children grow quickly.

Japanese can say that too:

• 子供は早く成長する。

But Japanese also very naturally uses noun phrases:

• 子供の成長は早い。
  literally, Children’s growth is fast.

So this sentence is a very normal Japanese way to express the idea.

It sounds like a general observation: children develop fast, and you notice how quickly they change.

Depending on context, it can sound like:

• a simple fact
• a parent or teacher noticing how quickly kids grow
• a reflective comment, like Kids grow up so fast

So even though the grammar is straightforward, the sentence can carry a feeling of surprise or realization depending on how it is used.