kodomotati ga tori wo oikakeruno wo mite, haha ha hasiranai you ni iimasita.

たち is a common plural-like suffix for people and animate beings. So 子供たち means the children rather than just a child or children in general.

A few important points:

• たち is not always required to make something plural in Japanese.
• Japanese often leaves number vague unless it matters.
• Here, 子供たち makes it clear that more than one child is involved.

So the sentence specifically points to the children as a group.

In this sentence, 子供たちが鳥を追いかけるのを見て is a subordinate clause: seeing the children chase a bird.

Inside that clause, 子供たち is the subject of 追いかける, so が is the normal choice.

Compare the roles:

• 子供たちが = the children are the ones doing the chasing
• 母は = as for the mother, she did the main action of the sentence

So:

• が marks the subject inside the embedded clause
• は marks the topic of the main sentence

Using 子供たちは here would shift the nuance and feel less neutral in this structure.

Because 鳥 is the direct object of 追いかける.

• 追いかける = to chase
• 鳥を追いかける = to chase a bird / the bird

So in the smaller clause:

• 子供たちが = the children
• 鳥を = the bird
• 追いかける = chase

That whole chunk then gets turned into something that can be the object of 見て.

Here, の nominalizes the clause before it. That means it turns 子供たちが鳥を追いかける into the act/event of the children chasing a bird.

So:

• 子供たちが鳥を追いかける = the children chase a bird
• 子供たちが鳥を追いかけるの = the children chasing a bird / the fact that the children are chasing a bird

This is very common with verbs of perception like:

• 見る = see
• 聞く = hear
• 知る = know in some contexts

In this sentence, the mother saw that event.

They belong to different verbs and mark different objects.

1. 鳥を goes with 追いかける

   • 鳥を追いかける = chase the bird

2. 追いかけるのを goes with 見る

   • 〜のを見る = see someone doing something / see the event of something happening

So the structure is:

• [子供たちが 鳥を 追いかける] のを 見て
• seeing [the children chase a bird]

The first を is inside the embedded action. The second を marks the whole embedded action as the thing being seen.

見て is the て-form, which links this action to the main action that follows.

So the sentence flows like this:

• 子供たちが鳥を追いかけるのを見て、
• 母は走らないように言いました。

In English, this is like:

• Seeing the children chase a bird, the mother told them not to run.

The て-form often means:

• and
• after doing
• because
• when

Here it gives the sense of after/when seeing that, then the mother spoke.

The sentence does not explicitly say to whom the mother spoke. Japanese often leaves that out when it is obvious from context.

From the meaning, we naturally understand that she told the children not to run.

If the sentence wanted to state it clearly, it could say something like:

• 母は子供たちに走らないように言いました。

Here:

• 子供たちに = to the children

But omitting it is very natural when the listener can easily infer it.

This pattern means so as not to run or, with verbs like 言う, to tell someone not to run.

Break it down:

• 走る = to run
• 走らない = not run
• 走らないように = so that someone does not run / not to run

With 言う, the pattern often expresses an instruction, advice, or warning:

• 〜ように言う = tell someone to do ~
• 〜ないように言う = tell someone not to do ~

So:

• 走らないように言いました = told them not to run

Both can mean told them not to run, but the tone is different.

• 走るな is a direct negative command: Don’t run!
• 走らないように言いました is softer and more indirect: She told them not to run

So 〜ないように言う often reports instructions in a gentler or more explanatory way.

Compare:

• 母は「走るな」と言いました。
  The mother said, Don’t run.
• 母は走らないように言いました。
  The mother told them not to run.

The sentence you were given sounds more natural in ordinary narration.

Before の in this kind of embedded clause, Japanese normally uses the plain form, not the polite ます-form.

So:

• 追いかけるのを見る = see someone chase
• not 追いかけますのを見る

This is a general rule: clauses placed inside larger sentences usually use plain forms, even if the main sentence ends politely.

That is why the sentence has:

• embedded clause: 追いかける
• main sentence ending politely: 言いました

Usually, with 見る, の is much more natural than こと.

• 〜のを見る is the normal pattern for directly perceiving an action or event.
• こと is more abstract and often less natural with immediate visual perception.

So:

• 子供たちが鳥を追いかけるのを見て = natural
• 子供たちが鳥を追いかけることを見て = unnatural or at least much less standard

As a rough guide:

• の often works well for something seen or heard as an event
• こと often works better for abstract facts, rules, or concepts

母は marks the mother as the topic of the main sentence.

The structure is:

• subordinate clause: 子供たちが鳥を追いかけるのを見て
• main clause: 母は走らないように言いました

Using は here tells us that the sentence is now about what the mother did in response.

If you used 母が, it would put more focus on the mother as the subject in a contrastive or identifying way, such as when answering Who said that?

So 母は is the more neutral narrative choice.

Grammatically, 追いかける means to chase or to run after / go after.

Which English version sounds best depends on context:

• 鳥を追いかける = chase a bird
• sometimes run after a bird if the physical running is important

In this sentence, because the next part says the mother told them not to run, the idea of physical chasing is important. So English might naturally use running after the bird even though the Japanese verb itself is 追いかける.

The sentence itself does not force a very precise distinction. 〜のを見る often means perceiving an action/event, and in practice it can cover either:

• seeing the action happen
• noticing that it is happening

So 子供たちが鳥を追いかけるのを見て can be understood as:

• when she saw the children chasing a bird
• after noticing the children were chasing a bird

Japanese often leaves that level of precision to context.

Both are possible in some contexts, but they give slightly different views of the action.

• 追いかけるのを見る can describe seeing the event/action in a general way
• 追いかけているのを見る emphasizes that the action is in progress

So:

• 子供たちが鳥を追いかけるのを見て = seeing the children chase / seeing the children chasing
• 子供たちが鳥を追いかけているのを見て = seeing that the children were in the middle of chasing the bird

The version without いる is perfectly natural and a bit simpler and less explicitly progressive.