Ta zjeżdżalnia jest wyższa niż tamta, więc mały chłopiec z początku trochę się bał.

Because they all refer to zjeżdżalnia, which is a feminine singular noun.

In Polish, words like demonstratives and adjectives must agree with the noun in gender, number, and case:

• ta zjeżdżalnia = this slide
• wyższa = taller / higher and matches the feminine noun
• tamta = that one, also feminine because it still refers to zjeżdżalnia

So the whole comparison stays feminine.

• ta means this
• tamta means that

So:

• ta zjeżdżalnia = this slide
• tamta = that one / that slide

Tamta often feels like that other one over there. In this sentence, it contrasts one slide with another.

Wyższa is the comparative form of wysoka / wysoki = high / tall.

So:

• wysoka = high
• wyższa = higher

This is the normal Polish way to say higher/taller here.
Using bardziej wysoka would sound unnatural in this sentence. Polish usually prefers the simple comparative form when one exists.

Also notice the stem change:

• wysoki → wyższy
• feminine: wyższa

Niż means than and is commonly used after a comparative:

• wyższa niż tamta = higher than that one

Yes, you could also use od:

• Ta zjeżdżalnia jest wyższa od tamtej.

Both mean the same thing. The main difference is grammatical:

• after niż: tamta
• after od: tamtej (genitive case)

So a learner should recognize both patterns:

• wyższa niż tamta
• wyższa od tamtej

Because Polish often leaves out a noun when it is obvious from context.

So:

• wyższa niż tamta literally means higher than that one
• the missing noun is understood: tamta zjeżdżalnia

This works just like English this one / that one.

Więc means so, therefore, or as a result.

It connects the two ideas:

• the slide is higher than the other one
• so the little boy was a bit afraid at first

It is a very common linking word in Polish.

Z początku means at first or initially.

So:

• z początku trochę się bał = at first he was a little afraid

This is slightly different from na początku, which more literally means at the beginning of something.

Very roughly:

• z początku = initially, at first
• na początku = at the beginning

In many contexts they overlap, but z początku is especially natural when talking about an initial reaction that later changes.

Trochę means a little, a bit, or somewhat.

It softens the statement:

• bał się = he was afraid
• trochę się bał = he was a little afraid / somewhat afraid

So the sentence does not say he was terrified, only that he felt some fear.

Yes. The basic verb is bać się = to be afraid.

So się is part of the verb expression. It is not really translated as himself here. You should learn it as one unit:

• bać się = to be afraid
• boję się = I am afraid
• bał się = he was afraid

Word order can vary:

• trochę się bał
• trochę bał się

Both are possible, though trochę się bał sounds very natural.

Also, when this verb has an object, it often takes the genitive:

• bać się psa = to be afraid of the dog
• bać się tej zjeżdżalni = to be afraid of this slide

Because Polish past tense agrees with the subject in gender and number.

Here the subject is mały chłopiec, which is masculine singular, so:

• bał się = he was afraid

Compare:

• dziewczynka bała się = the girl was afraid
• chłopcy bali się = the boys were afraid
• dziewczynki bały się = the girls were afraid

So the ending tells you something about who is doing the action.

Because Polish often drops subject pronouns when they are clear from context.

English needs he, but Polish often does not.

Here the subject is already given:

• mały chłopiec ... się bał

So adding on would usually be unnecessary.

In this context, it most naturally means little boy.

The adjective mały can mean:

• physically small
• young / little

With chłopiec, English usually prefers little boy unless the speaker specifically wants to emphasize physical size.