Všechny děti se těší na prázdniny.

It has three main parts:

• Všechny děti = all the children
• se těší = are looking forward / look forward
• na prázdniny = to the holidays / vacation

So the structure is:

• subject: Všechny děti
• verb: se těší
• prepositional phrase: na prázdniny

A very literal version would be:

• All children are looking forward to the holidays.

Because všechny agrees with děti.

The noun děti is the plural of dítě = child, and it behaves irregularly. With děti, Czech normally uses adjective forms like:

• malé děti = small children
• hodné děti = well-behaved children
• všechny děti = all children

So všechny děti is the normal, correct phrase.

A learner may expect something else because dítě is singular neuter, but its plural is special, and you simply learn the pattern všechny děti.

Děti means children.

Its singular is:

• dítě = child

This is an irregular noun:

• singular: dítě
• plural: děti

So the sentence is talking about more than one child.

Because těšit se is a reflexive verb.

In this sentence, you should learn the whole expression:

• těšit se na něco = to look forward to something

So se is not optional here. You cannot normally say just děti těší na prázdniny.

This is similar to learning a verb together with a required particle or preposition in English. It is best to treat těšit se na as one unit of meaning.

Here těšit se means to look forward to.

So:

• těšit se na prázdniny = to look forward to the holidays

Be careful: the verb těšit can appear in other patterns too, with different meanings. For example:

• Těší mě. = It pleases me. / Nice to meet you in some contexts.

But in your sentence, the pattern is specifically:

• těšit se na + accusative = to look forward to

In Czech, some verbs have the same form for he/she/it and they.

With těšit se, the present tense looks like this:

• těším se = I look forward
• těšíš se = you look forward
• těší se = he/she/it looks forward
• těšíme se = we look forward
• těšíte se = you look forward
• těší se = they look forward

So těší se can mean either:

• he/she/it is looking forward, or
• they are looking forward

In your sentence, the subject všechny děti makes it clear that it is plural.

Because the expression is:

• těšit se na + accusative

So prázdniny is in the accusative here.

The full pattern is:

• těšit se na něco = to look forward to something

Even though prázdniny looks the same in nominative and accusative plural, its role here is accusative because it follows na in this expression.

Yes, prázdniny is a plural form, and in normal usage it refers to school holidays, vacation, or break.

Czech often uses this plural word where English may use either:

• the holidays
• vacation
• school break

For example:

• Máme prázdniny. = We’re on vacation / It’s holiday time.
• Těším se na prázdniny. = I’m looking forward to the holidays.

So even if English sometimes uses a singular word, Czech naturally uses prázdniny.

Because the feeling is happening now.

The children are now in a state of looking forward to something that will happen later. So Czech uses the present tense naturally:

• Všechny děti se těší na prázdniny.

This is very similar to English:

• The children are looking forward to the holidays.

The holidays are future, but the emotion exists in the present.

Yes. Czech word order is flexible, and changing it usually changes emphasis, not the core meaning.

The neutral version is:

• Všechny děti se těší na prázdniny.

Other possible orders include:

• Na prázdniny se těší všechny děti.
  Emphasis on the holidays or on the whole idea of what they are looking forward to.

• Všechny děti na prázdniny se těší.
  Grammatically possible, but less neutral and more marked.

For a learner, the original sentence is the most natural basic version.

Here děti is in the nominative plural because it is the subject of the sentence.

You can ask:

• Who is looking forward to the holidays?
• All the children.

That tells you děti is the subject, so nominative is the correct case.

That would be:

• Dítě se těší na prázdniny.

Compare:

• Dítě se těší na prázdniny. = The child is looking forward to the holidays.
• Děti se těší na prázdniny. = The children are looking forward to the holidays.
• Všechny děti se těší na prázdniny. = All the children are looking forward to the holidays.

This is a useful way to see how the plural works.