Kad se vratimo kući, djeca će sigurno crtati konja, kravu i traktor.

Because after kad meaning when, Croatian often uses the present tense of a perfective verb to refer to a future completed action.

So:

• Kad se vratimo kući = When we get back home / When we return home

Even though vratimo looks like present tense, here it refers to a future event. This is very common in Croatian.

Compare:

• Kad dođem, nazvat ću te. = When I arrive, I’ll call you.
• Kad završimo, idemo kući. = When we finish, we’re going home.

Using a future form like kad ćemo se vratiti would normally sound wrong in this kind of sentence.

The verb is vratiti se, which means to return / to come back.

The little word se is part of the verb. Without it, vratiti usually means to return something or to give something back, not to return oneself.

So:

• vratiti se = to return, come back
• vratiti knjigu = to return a book

In your sentence:

• Kad se vratimo kući = When we return home

Kući is a very common way to say home when expressing movement toward home.

So:

• ići kući = to go home
• vratiti se kući = to return home

This form is extremely common and is often learned as a fixed expression.

You may also see:

• u kuću = into the house

But that emphasizes going into a building, while kući usually means home in the broader sense.

So in this sentence, kući is the natural choice.

That is the normal way to form the future tense in Croatian.

Future I is usually:

• ću, ćeš, će, ćemo, ćete, će
  • infinitive

Here:

• djeca će crtati = the children will draw / will be drawing

So:

• će = future auxiliary
• crtati = infinitive, to draw

With some verbs, especially in writing, the infinitive may lose the final -i when it comes after the auxiliary:

• pisati → pisat ću
• raditi → radit ću

But with će + infinitive, the form in your sentence is completely standard:

• će crtati

This is about aspect, which is very important in Croatian.

• crtati = imperfective
• nacrtati = perfective

Very roughly:

• crtati focuses on the activity/process: to draw, to be drawing
• nacrtati focuses on completion/result: to draw something completely, to finish drawing

So:

• djeca će crtati suggests the action/activity of drawing
• djeca će nacrtati would suggest they will complete the drawings

In your sentence, crtati fits well if the idea is simply that the children will be occupied with drawing.

Yes, djeca means children, so it is plural in meaning.

The form će does not show singular or plural by itself in a way that helps here, because će is the same for:

• he/she/it will
• they will

So djeca će crtati is perfectly correct:

• The children will draw

A useful thing to know is that djeca is a somewhat irregular noun. It means more than one child, but its form does not look like a typical plural noun such as psi or auti.

Still, it takes plural agreement in sentences:

• Djeca su vani. = The children are outside.
• Djeca crtaju. = The children are drawing.

Because they are direct objects, so they are in the accusative case.

Here is what happens:

• konj → konja
• krava → kravu
• traktor → traktor

Why are they different?

1. konja

Konj is masculine animate, and masculine animate nouns usually have accusative singular equal to the genitive singular:

• konj → konja

2. kravu

Feminine nouns ending in -a usually change to -u in the accusative singular:

• krava → kravu

3. traktor

Traktor is masculine inanimate, and masculine inanimate nouns usually have accusative singular the same as nominative:

• traktor → traktor

So the sentence gives you three very useful accusative patterns at once.

Because konj is a masculine animate noun, and masculine animate nouns usually change in the accusative singular.

So:

• nominative: konj = horse
• accusative: konja = horse as a direct object

Since the children are drawing the horse, horse is the direct object, so Croatian uses the accusative:

• crtati konja

Compare:

• Vidim konja. = I see a horse.
• Konj je velik. = The horse is big.

In the second sentence, konj is the subject, so it stays nominative.

Croatian has no articles like English a/an and the.

So:

• konj can mean a horse or the horse
• krava can mean a cow or the cow
• traktor can mean a tractor or the tractor

The exact meaning depends on context.

In your sentence, konja, kravu i traktor could be understood in English as:

• a horse, a cow, and a tractor or
• the horse, the cow, and the tractor

Usually English translations choose what sounds most natural in context.

Sigurno here is an adverb meaning surely, certainly, or probably/very likely, depending on context.

In this sentence:

• djeca će sigurno crtati... = the children will surely / certainly be drawing...

Croatian word order is flexible, so sigurno can often move around:

• Djeca će sigurno crtati.
• Djeca sigurno će crtati.
• Sigurno će djeca crtati.

Some versions are more natural than others depending on style and emphasis. The version in your sentence is perfectly normal.

Yes, in many contexts doma can also mean home.

So both are common:

• vratiti se kući
• vratiti se doma

However, usage can vary by region and style. Kući is extremely standard and very widely accepted. If you are learning standard Croatian, kući is always a safe choice.

The sentence has two parts:

1. Kad se vratimo kući
   = a time clause: When we get home

2. djeca će sigurno crtati konja, kravu i traktor
   = the main clause: the children will surely be drawing a horse, a cow, and a tractor

So the structure is:

• [time clause], [main clause]

This is very natural in Croatian, just as in English:

• When we get home, the children will...

You could also put the main clause first in some contexts, but the version given is the most straightforward.

Then you would probably switch from crtati to a perfective verb such as nacrtati.

For example:

• Kad se vratimo kući, djeca će sigurno nacrtati konja, kravu i traktor.

That sounds more like:

• When we get home, the children will surely draw/finish drawing a horse, a cow, and a tractor.

The difference is:

• crtati = focus on the process/activity
• nacrtati = focus on the completed result

This aspect difference is one of the most important things to notice in Croatian verbs.