qǐng gēn háizi shuō wǎn'ān.

请 makes the sentence polite. Here, it works like please in English.

So 请跟孩子说晚安。 is a polite request or instruction:

• Please say goodnight to the child.

In other contexts, 请 can also mean to invite or to treat someone, but here it is just the politeness marker at the start of a request.

In this sentence, 跟 introduces the person you are speaking to. It often means with, but in patterns like this it can mean something like to when used with verbs of speaking.

So:

• 跟孩子说晚安 = say goodnight to the child

A useful pattern is:

• 跟 + person + 说 + something

Examples:

• 跟老师说谢谢。 — say thank you to the teacher
• 跟朋友说再见。 — say goodbye to a friend

Chinese often puts the target of the action before the main verb when using words like 跟, 对, or 给.

So the structure is:

• 请 + 跟 + person + 说 + words

Literally, it is something like:

• Please + to/with the child + say + goodnight

This is normal Chinese word order.
English says say goodnight to the child, but Chinese commonly says to the child say goodnight.

说 is needed because 晚安 is the thing being said.

• 说晚安 = say goodnight

Without 说, 跟孩子晚安 does not sound natural in standard Mandarin.

Compare:

• 跟孩子说晚安。 — say goodnight to the child
• 跟孩子说谢谢。 — say thank you to the child

So 说 connects the act of speaking with the words being spoken.

It can mean either child or children, depending on context.

Chinese nouns usually do not have to show singular vs. plural as clearly as English nouns do. So:

• 孩子 can mean the child or the children

If the speaker wants to be clearly plural, they might say:

• 孩子们 — children

But 孩子 by itself is very common and natural.

The second syllable 子 is neutral-toned here.

So it is pronounced:

• háizi

not

• háizǐ

This is very common in everyday Mandarin. Some words have a full tone in writing for an individual character, but in a real word the syllable becomes neutral.

Other examples:

• 桌子 → zhuōzi
• 名字 → míngzi

So when learning vocabulary, it is best to learn the pronunciation of the whole word, not only the tones of the individual characters.

The apostrophe helps separate syllables in pinyin.

• 晚安 = wǎn’ān

It is written with an apostrophe because the second syllable starts with a. Without the apostrophe, it could be harder to see where one syllable ends and the next begins.

This is just a spelling aid in pinyin. It does not mean a pause or a special sound in Mandarin speech.

Similar examples:

• 西安 = Xī’ān
• 天安门 = Tiān’ānmén

Yes, in many situations 对 can also work:

• 请对孩子说晚安。

This is grammatical and understandable.
However, 跟孩子说晚安 often sounds a bit more natural and conversational.

Very roughly:

• 跟 feels more like to/with
• 对 feels a little more explicitly directed toward

Both are common with 说, but learners will hear 跟 someone 说... very often in everyday speech.

The subject is omitted, which is very common in Chinese.

The full idea is something like:

• (你) 请跟孩子说晚安。
• (You) please say goodnight to the child.

Chinese often leaves out subjects when they are obvious from context, especially in commands, requests, and conversation.

Yes, it is natural. It sounds like a polite instruction or request.

For example, someone might say this:

• to a parent
• to a teacher
• to a family member
• in a childcare setting

It is slightly more complete than just saying:

• 跟孩子说晚安。

Adding 请 makes it more polite.

Yes, that is possible, but it sounds more formal or literary.

• 说晚安 — more everyday and straightforward
• 道晚安 — more formal, more elegant, less common in ordinary speech

For most learners, 跟孩子说晚安 is the most useful and natural version to remember.

Usually yes. 晚安 is specifically good night, not just good evening.

So you say it when:

• someone is going to bed
• you are ending the night
• you want to wish someone a good night’s rest

It is not the same as greeting someone in the evening. For that, Mandarin more often uses:

• 晚上好 — good evening

So in this sentence, 说晚安 really means giving a bedtime or end-of-night wish.