umi he iku mae ni, kodomo ni hiyakedome wo nutte agemasu.

Because 海へ行く is modifying 前.

In Japanese, when a verb comes before a noun-like word such as 前 (before), it normally appears in the plain form, not the ます form. So:

• 海へ行く前に = before going to the sea / beach

Using 行きます前に would be ungrammatical.

前に means before.

The pattern is:

• Verb dictionary form + 前に = before doing ...

So:

• 海へ行く前に = before going to the sea / beach

This is a very common grammar pattern.

へ marks direction: toward or to.

So 海へ行く means go to/toward the sea. With verbs like 行く, both へ and に are often possible:

• 海へ行く
• 海に行く

Both are natural. Very roughly:

• へ emphasizes direction
• に emphasizes destination

In many everyday sentences, the difference is small.

That に is part of the grammar pattern 前に.

It is not doing the same job as the に in 子供に. These are two different uses:

• 海へ行く前に = before going to the sea
• 子供に = for the child / to the child

So even though they are both に, they are marking different things.

Because 子供 is not the direct object here. The direct object is 日焼け止め.

• 日焼け止めをぬる = apply sunscreen
• 子供に ... ぬってあげる = apply it for the child

So:

• 日焼け止めを = the thing being applied
• 子供に = the person receiving the favor/action

That is why に is used for 子供.

あげます adds the meaning of doing something for someone.

So:

• 日焼け止めをぬります = I apply sunscreen
• 日焼け止めをぬってあげます = I apply sunscreen for someone

Here it shows that the speaker is doing the action as a helpful act for the child.

This is called a benefactive expression. It often carries a nuance of kindness, care, or service.

Because ぬって is the て-form of ぬる.

ぬる is a -u verb, and its て-form is irregular in the usual way for this verb type:

• ぬる → ぬって

The て-form is needed before あげます:

• ぬってあげます = apply it for someone

So the sentence is built from:

• 日焼け止めをぬる
  • あげます
• which becomes 日焼け止めをぬってあげます

Yes. ぬる means to spread/apply something onto a surface, and with 日焼け止め it means to apply sunscreen onto the skin.

It is commonly used for things like:

• cream
• lotion
• sunscreen
• medicine/ointment

So this is the natural verb for putting sunscreen on someone.

It could be either. Japanese nouns usually do not show singular/plural unless the context makes it clear.

So 子供に can mean:

• for a child
• for the child
• for the children

The exact number depends on context.

Japanese often leaves out information that is obvious from context.

Here, 日焼け止めをぬる already strongly suggests applying it to the skin, so the body part does not need to be stated. If needed, you could add it:

• 顔に日焼け止めをぬってあげます = I apply sunscreen to the child’s face
• 腕に日焼け止めをぬってあげます = I apply sunscreen to the child’s arms

But without that extra detail, the original sentence is completely natural.

Not exactly lower, but あげる is used when the speaker gives a favor to someone else.

In family situations, with children, friends, or people close to you, 〜てあげます is very common and natural. Here it fits well because the speaker is caring for the child.

One nuance to remember: talking about doing favors for yourself or your in-group can sometimes sound a little self-congratulatory depending on context. But in a simple sentence like this, it is a normal grammar example.

Formally, あげます is non-past, so it can mean either present/habitual or future, depending on context.

So this sentence could mean things like:

• I apply sunscreen to the child before going to the beach
  (habitual/general)
• I will apply sunscreen to the child before going to the beach
  (future plan)

Japanese often relies on context rather than changing the verb form for future meaning.