Если придут дети, мы дадим каждому по кусочку торта и по стакану сока.

This is a very common question. Russian and English handle future conditions differently.

In English, after if, we normally use a present form for future meaning: If the children come, we’ll give...

In Russian, it is normal to use a future form when the situation is really in the future:

Если придут дети, мы дадим...

So придут is exactly what you expect here. It is the future form of прийти and means will come / arrive.

Because придут refers to a single completed arrival in the future.

• придут = they will come / arrive
• приходят = they come, are coming, or come regularly, depending on context

Here the sentence is about one possible future event: the children may show up. That is why Russian uses прийти in the future: придут.

Because Если придут дети is a subordinate clause introduced by если.

Russian normally separates this kind of clause with a comma:

Если придут дети, мы дадим каждому...

This is similar to English punctuation in sentences like If the children come, we’ll give...

Yes. Both are possible.

• Если придут дети
• Если дети придут

Russian word order is flexible. The version in your sentence puts the action придут first, then the subject дети. That can sound a bit more natural in some contexts when introducing a possible event.

Если дети придут is also perfectly correct. It simply puts дети earlier.

Because дадим presents the action as a single complete act: we will give.

That fits the situation well: if the children come, we will hand out cake and juice once.

• дадим = perfective future, one complete action
• будем давать = imperfective future, more about process, repetition, or ongoing action

So дадим is the natural choice here.

Каждому is the dative singular form of каждый.

It means to each one or to each child.

The verb дать takes the person receiving something in the dative case:

• дать кому? = to give to whom?

So:

• каждому = to each one

You can think of it as short for каждому ребёнку.

Because каждому refers to each child individually.

The sentence is not treating the children as one group receiving one shared thing. It is distributing items one by one:

• дети = the children, as a group
• каждому = to each individual child

So singular is exactly right here.

Here по has a distributive meaning: one each, apiece.

So:

• каждому по кусочку торта = each gets a piece of cake
• по стакану сока = a glass of juice each

It helps make the distribution very clear.

Yes, you could, and it would still make sense.

But по makes the distributive meaning more explicit: each child gets one piece and one glass.

Compare:

• дадим каждому кусочек торта и стакан сока = we’ll give each child a piece of cake and a glass of juice
• дадим каждому по кусочку торта и по стакану сока = we’ll give each child one piece of cake each and one glass of juice each

In practice, both can work, but по strongly emphasizes apiece.

In this pattern, по is followed by the dative singular:

• кусочек → кусочку
• стакан → стакану

So:

• по кусочку
• по стакану

This is a very common structure in Russian:

• каждому по яблоку
• по билету
• по чашке чая

It means something like one ... for each person.

Because Russian usually puts the substance or whole thing in the genitive after a word meaning a part, amount, or container.

So:

• кусочек торта = a piece of cake
• стакан сока = a glass of juice

This is the same pattern as:

• чашка чая = a cup of tea
• бутылка воды = a bottle of water

Only the first noun changes after по:

• по кусочку торта
• по стакану сока

The second noun stays genitive.

The form кусочку is just the dative singular of кусочек.

As for meaning:

• кусок = a piece
• кусочек = a little piece / small piece

Кусочек is a diminutive form, so it sounds a bit softer or smaller. In this sentence, по кусочку торта suggests a small piece of cake each.

If you said по куску торта, that would also be grammatical, but it sounds a little less diminutive.

Not necessarily. Russian often omits subject pronouns when the verb already makes the subject clear.

Since дадим already means we will give, you could say:

Если придут дети, дадим каждому по кусочку торта и по стакану сока.

That is grammatical.

Including мы can make the sentence a bit clearer, fuller, or slightly more emphatic.