Στο νηπιαγωγείο τα παιδιά έχουν μάθει να μοιράζονται τις κηρομπογιές και την πλαστελίνη.

Στο is the normal contraction of σε + το.

• σε = in / at / to
• το = the for a neuter singular noun

So:

• σε το νηπιαγωγείο → στο νηπιαγωγείο

After σε, Greek uses the accusative. Here, νηπιαγωγείο is neuter singular, and its accusative looks the same as its nominative, so the form does not visibly change.

Greek word order is quite flexible. Putting Στο νηπιαγωγείο first sets the scene: At/In kindergarten, ...

A more neutral order would also be possible:

Τα παιδιά έχουν μάθει να μοιράζονται τις κηρομπογιές και την πλαστελίνη στο νηπιαγωγείο.

Starting with the place phrase gives it a bit more prominence.

Τα παιδιά means the children.

• singular: το παιδί = the child
• plural: τα παιδιά = the children

It is a neuter noun, which is very common in Greek for words referring to young people or children.

Because τα παιδιά is a plural subject, so the verb is plural too:

• το παιδί έχει = the child has
• τα παιδιά έχουν = the children have

So έχουν μάθει means they have learned.

Έχουν μάθει is the perfect form of the verb μαθαίνω (to learn).

• μαθαίνω = I learn / I am learning
• έμαθα = I learned
• έχω μάθει = I have learned

So τα παιδιά έχουν μάθει means the children have learned or the children have learned and now know how.

This perfect often emphasizes the result in the present. In other words, they learned it, and this is now part of what they can do.

Modern Greek does not use a normal infinitive the way English does. Instead, it usually uses να + verb form.

So where English says:

• have learned to share

Greek says:

• έχουν μάθει να μοιράζονται

Literally, this is something like they have learned that they share / to be sharing, but in natural English the meaning is simply they have learned to share.

Μοιράζονται is the 3rd person plural form of μοιράζομαι.

Even though it looks like a passive-form verb, here it means share, not are shared.

So:

• μοιράζομαι = I share
• μοιράζονται = they share

This is very common in Greek: some verbs use middle/passive endings but have an active meaning in English.

In this sentence, it has a natural sense of sharing among themselves / with each other.

Because the sentence is talking about a general ability or habit, not one single completed act.

• να μοιράζονται = to share, to be in the habit of sharing
• να μοιραστούν would sound more like to share on one particular occasion or to manage to share once

After μαθαίνω να..., the imperfective form is very natural when the meaning is learning a skill or behavior.

So έχουν μάθει να μοιράζονται means they have learned to share as a general social skill.

Because the two nouns are different in number, and the article must agree with each noun.

• τις κηρομπογιές = feminine plural accusative
• την πλαστελίνη = feminine singular accusative

These are direct objects of μοιράζονται, so they appear in the accusative.

You can think of it like this:

• κηρομπογιές = crayons → plural
• πλαστελίνη = plasticine / modeling clay → singular

Greek normally repeats the article when linking nouns, especially when the nouns differ in number or are felt as separate items.

So:

• τις κηρομπογιές και την πλαστελίνη

is the normal, natural phrasing.

It keeps the structure clear and sounds fully idiomatic. In English, we often omit the second article, but Greek commonly keeps it.

Because πλαστελίνη is usually treated as a mass noun, like clay or play-dough in English.

So Greek naturally says:

• την πλαστελίνη = the plasticine / the modeling clay

But crayons are separate countable items, so Greek uses the plural:

• τις κηρομπογιές = the crayons

Yes, but the meaning shifts slightly.

• έμαθαν να μοιράζονται = they learned to share
  This simply reports a past event.

• έχουν μάθει να μοιράζονται = they have learned to share
  This emphasizes the present result: they now know how / they now do this.

So in this sentence, έχουν μάθει is a good choice if the speaker wants to stress the children’s current social development.