Guardei algumas sementes do tomate para as mostrar à minha filha.

Guardei is the 1st person singular of the pretérito perfeito simples of guardar.

So it means I kept, I saved, or I put aside in the sense of a completed past action.

• guardar = to keep / to save / to store
• guardei = I kept / I saved

In this sentence, it shows that the speaker already did this action.

Because algumas means some, and it has to agree with sementes.

• semente = seed
• sementes = seeds
• algumas = some (feminine plural)

Since sementes is a feminine plural noun, the determiner also has to be feminine plural:

• algumas sementes

An English speaker may not expect this, because some never changes in English, but in Portuguese it does.

This is an important nuance.

• do tomate = from the tomato / of the tomato
• de tomate = tomato in a more general or descriptive sense

So:

• sementes do tomate suggests seeds taken from a specific tomato
• sementes de tomate would more naturally mean tomato seeds in general

Also, do is a contraction:

• de + o = do

So do tomate literally comes from de o tomate.

Because the seeds came from one tomato.

So the idea is:

• many seeds
• from one tomato

That is completely normal. The number of sementes does not have to match the number of tomate.

As means them.

It is a direct object pronoun, and it refers back to algumas sementes.

Why as?

• sementes is feminine plural
• the matching direct object pronoun is as

So:

• sementes → as
• para as mostrar = to show them

Because in European Portuguese, after a preposition such as para, it is very common and very natural to place the unstressed object pronoun before the infinitive:

• para as mostrar

This is a pattern learners of Portuguese from Portugal should get used to.

So in Portugal, para as mostrar sounds very natural.
An attached form such as para mostrá-las may also be seen or understood, but para as mostrar is the more characteristically European Portuguese pattern here.

Because mostrar normally works as mostrar alguma coisa a alguém = to show something to someone.

Here:

• as = the thing being shown
• à minha filha = the person receiving what is shown

The à is a contraction of:

• a (the preposition required by mostrar)
• a (the feminine singular article before minha filha)

So:

• a + a = à

And in European Portuguese, possessives very often come with the definite article:

• a minha filha = my daughter

So after contraction:

• à minha filha

It could be.

Portuguese can sometimes leave out the object if it is obvious from the context. Since algumas sementes do tomate has just been mentioned, a listener can understand what is being shown.

So both ideas are possible:

• para mostrar à minha filha = to show my daughter
• para as mostrar à minha filha = to show them to my daughter

The version with as is simply more explicit: it clearly repeats the seeds as the thing being shown.

Yes, slightly.

English often says:

• show my daughter the seeds
• show the seeds to my daughter

Portuguese very naturally uses:

• mostrar as sementes à minha filha

So the structure is often:

• mostrar + thing + a someone

In the sentence, the thing is replaced by the pronoun as, giving:

• mostrar as sementes à minha filha
• mostrar as à minha filha → not possible
• mostrar-lhes as sementes → different structure
• para as mostrar à minha filha → correct here

So the sentence keeps the Portuguese pattern of show something to someone.