Kad budem imala više vremena, sastavit ću poruku za roditelje.

Because after kad meaning when in a future-time clause, Croatian normally uses future II in the subordinate clause.

So the pattern is often:

• Kad + future II, future I
• Kad budem imala više vremena, sastavit ću poruku...

This corresponds to English When I have more time, I’ll draft the message...

Using Kad ću imati... is generally not the normal standard choice in this kind of sentence.

Budem imala is future II.

It is formed with:

• budem = a form of biti (to be)
• imala = the l-participle of imati (to have), feminine singular

So:

• budem imala = I will have
• literally, something like I will be having / I will have come to have, but in practice just I have / will have in a future clause

Future II is very common after words like kad (when), ako (if), čim (as soon as), and similar conjunctions.

Because the speaker is understood to be female.

In Croatian, the l-participle agrees with the gender of the subject:

• budem imala = if the speaker is female
• budem imao = if the speaker is male

So a male speaker would say:

• Kad budem imao više vremena, sastavit ću poruku za roditelje.

Because the sentence has two clauses with different roles:

1. Kad budem imala više vremena = the time condition / subordinate clause
2. sastavit ću poruku za roditelje = the main clause

In Croatian, when the first clause refers to a future situation introduced by kad (when), it usually uses future II.
The main clause then uses future I.

So:

• Kad budem imala... = When I have...
• sastavit ću... = I will draft...

This is a very normal Croatian pattern.

In standard Croatian, the future with ću, ćeš, će... is written separately.

With an infinitive ending in -ti, when ću comes after it, the final -i drops in writing:

• sastaviti + ću → sastavit ću
• napisati + ću → napisat ću

So sastavit ću is the standard Croatian spelling.

Writing it as one word, sastaviću, is not standard Croatian orthography.

Yes. That is also correct.

You can express the future in two common ways:

• sastavit ću poruku
• ću sastaviti poruku

Both are grammatical.

The version in your sentence, sastavit ću, is very common and natural.
If you want extra emphasis on the subject, you could also say:

• Ja ću sastaviti poruku za roditelje.

Croatian word order is flexible, but not random; different orders can shift emphasis slightly.

Because after quantity expressions like više (more), Croatian usually uses the genitive.

So:

• vrijeme = nominative singular, basic dictionary form
• vremena = genitive singular

That is why you get:

• više vremena = more time

This is very common:

• puno vremena = a lot of time
• malo vremena = little time
• dosta vremena = enough time

Because poruku is the direct object of sastaviti.

The verb sastaviti here means something like to compose, to draft, or to put together, and it takes an object in the accusative.

So:

• poruka = nominative singular
• poruku = accusative singular

That is why the sentence has:

• sastavit ću poruku = I will draft a message

Roditelje is accusative plural.

The preposition za often takes the accusative, especially when it means for.

So:

• roditelji = parents, nominative plural
• roditelje = accusative plural

Thus:

• za roditelje = for the parents / for parents

Yes, but it would mean something slightly different.

• poruka za roditelje = a message for parents, a message intended for them as the audience
• poruka roditeljima = a message to the parents, emphasizing them as recipients

In many contexts these are very close in meaning, and both can sound natural.
Your sentence with za roditelje suggests the message is meant for parents as a target audience, for example a school message intended for parents.

Sastaviti poruku means something like:

• to draft a message
• to compose a message
• to put together a message

It suggests creating or formulating the content.

Yes, napisati poruku is also possible and very common:

• Kad budem imala više vremena, napisat ću poruku za roditelje.

The difference is roughly:

• napisati = write
• sastaviti = compose / draft / formulate

In many everyday situations, either works.

Yes, sastaviti is a perfective verb.

That means it presents the action as a completed whole:

• sastavit ću poruku = I will draft/finish composing the message

This fits well with a one-time completed future action.

By contrast, imati is imperfective, which makes sense because having time is a state rather than a completed event.

So the sentence combines:

• budem imala više vremena = when I am in the situation of having more time
• sastavit ću poruku = I will complete the act of drafting the message

Yes, it is natural and correct.

It sounds like something a female speaker might say in a fairly neutral or slightly formal context, especially if she plans to prepare a message intended for parents.

A few natural variants would be:

• Kad budem imala više vremena, napisat ću poruku za roditelje.
• Kad budem imala više vremena, sastavit ću poruku roditeljima.

But your original sentence is fully idiomatic.