Djeca crtaju kredom po pločniku i pušu velike mjehuriće.

Djeca means children. It is the irregular plural of dijete (child).

So even though the form may not look obviously plural to an English speaker, it is plural in meaning, and that is why the verbs are plural too: crtaju and pušu.

• dijete = child
• djeca = children

Both are present tense, 3rd person plural verb forms.

• crtaju = they draw / are drawing
• pušu = they blow / are blowing

They match the plural subject djeca.

In this kind of sentence, Croatian uses the present tense to describe an action happening now or a general scene.

Because the dictionary form is puhati (to blow), but its present tense stem changes:

• puhati
• pušem
• pušeš
• puše
• pušemo
• pušete
• pušu

So pušu is the correct present plural form.
By contrast, crtati is more regular:

• crtam
• crtaš
• crta
• crtamo
• crtate
• crtaju

Because Croatian uses the instrumental case here.

Kredom means with chalk or using chalk. When you want to show the tool or means used to do something, Croatian often uses the instrumental without a preposition.

Compare:

• pisati olovkom = to write with a pencil
• rezati nožem = to cut with a knife
• crtati kredom = to draw with chalk

So:

• kreda = chalk
• kredom = with chalk

Here po is the natural choice because it suggests doing something over/on/across a surface.

So crtati po pločniku means drawing on the pavement/sidewalk surface.

If you said na pločniku, that would more strongly emphasize simple location: on the sidewalk.
With crtati, po is very common when talking about drawing on a surface like a wall, notebook, desk, or pavement.

So the difference is roughly:

• na pločniku = on the sidewalk
• crtati po pločniku = draw on/over the sidewalk surface

Because the preposition po here takes the locative case.

So:

• pločnik = sidewalk, pavement
• po pločniku = on/over the sidewalk

The ending changes to show the case:

• nominative: pločnik
• locative: pločniku

This is a very common pattern in Croatian: prepositions often force a specific case.

Because velike mjehuriće is the accusative plural, which is needed here because it is the direct object of pušu.

They are blowing what?
→ velike mjehuriće

Compare:

• nominative plural: veliki mjehurići = big bubbles
• accusative plural: velike mjehuriće = big bubbles, as the object

The adjective also changes to match the noun in case and number:

• veliki → nominative plural
• velike → accusative plural here

Because Croatian has no articles.

There is no direct equivalent of English the or a/an in ordinary Croatian grammar. Whether something is definite or indefinite is usually understood from context.

So djeca can mean:

• children
• the children
• sometimes some children

And the same idea applies to the other nouns in the sentence.

Because the same subject continues for both verbs.

Croatian, like English, usually does not repeat the subject if it is already clear:

• Djeca crtaju ... i pušu ...

This means:

• The children are drawing ... and blowing ...

You could repeat the subject, but normally that would sound heavier or add emphasis:

• Djeca crtaju ... i djeca pušu ...

So the version in the sentence is the normal, natural one.

Yes. Croatian word order is fairly flexible because the case endings already show who is doing what.

The original sentence has a neutral, natural order:

• Djeca crtaju kredom po pločniku i pušu velike mjehuriće.

But other orders are possible, especially if you want to emphasize one part:

• Djeca pušu velike mjehuriće i crtaju kredom po pločniku.
• Kredom djeca crtaju po pločniku i pušu velike mjehuriće.

Not every rearrangement sounds equally natural, but Croatian does allow much more flexibility than English.

A few useful approximations:

• š sounds like sh in ship
• č sounds like ch in church
• ć is a softer sound than č; English does not have a perfect match
• j sounds like y in yes

So for example:

• pušu ≈ POO-shoo
• pločniku ≈ PLOCH-nee-koo
• mjehuriće has a soft ć near the end, softer than ordinary English ch

The most important thing at first is to clearly distinguish:

• š from s
• č from ć
• j from English j

That will make your Croatian much easier to understand.