Wakiwa wamevaa sandaali na kaptula, watoto wanakimbia hadi bustanini.

Because they all agree with watoto (children), which is a plural noun for people.

In Swahili, verbs usually show agreement with the subject. For plural human nouns like watoto, the subject prefix is wa-.

So:

• wa-ki-wa → wakiwa
• wa-me-vaa → wamevaa
• wa-na-kimbia → wanakimbia

That wa- is basically telling you they.

Wakiwa means something like while they are, when they are, or being.

It is built from:

• wa- = they
• -ki- = a marker often used for while / when / if
• -wa = from kuwa, to be

So wakiwa wamevaa... means while they are wearing... or more literally while they are being in a state of having worn...

This kind of clause is very common in Swahili for giving background information.

Because in Swahili, kuvaa often works a little differently from English to wear.

Wamevaa is grammatically a perfect form:

• wa- = they
• -me- = perfect marker
• vaa = wear / put on

Literally, it is closer to they have put on. But in natural Swahili, this often describes the present result of that action, so it means they are wearing.

This is a very common pattern:

• amevaa shati = he/she is wearing a shirt
• literally: he/she has put on a shirt

Usually not. These are loanwords, and many Swahili loanwords keep the same form in both singular and plural.

So:

• sandali / sandaali can mean sandal or sandals, depending on context
• kaptula can mean shorts

The rest of the sentence tells you the subject is plural, so the overall meaning is clear.

Here na simply means and, joining the two clothing items:

• sandaali na kaptula = sandals and shorts

If you said wakiwa na sandaali na kaptula, that would more literally mean while they have sandals and shorts, not necessarily while they are wearing them.

To express wearing, Swahili normally uses kuvaa:

• wamevaa sandaali na kaptula = they are wearing sandals and shorts

So the sentence is using the more natural verb for clothing.

It can mean either, depending on context.

The -na- tense marker often covers:

• present ongoing action: are running
• general/habitual action: run

So watoto wanakimbia could mean:

• the children are running
• the children run

In this sentence, because it describes a scene, English usually translates it as are running.

Hadi means until, up to, as far as, or all the way to, depending on context.

With movement toward a place, it often means:

• up to
• as far as
• all the way to

So hadi bustanini means something like:

• to the garden
• as far as the garden
• all the way to the garden

Because -ni is a locative ending.

Adding -ni to a noun often gives the idea of:

• in
• at
• to a place.

So:

• bustani = garden / park
• bustanini = in/at/to the garden/park

Since the verb is wanakimbia (they run), the locative form fits naturally: they are running to or in that place, depending on context.

It is there to set the scene first.

Swahili often puts this kind of background clause before the main clause:

• Wakiwa wamevaa sandaali na kaptula, watoto wanakimbia hadi bustanini.

This is similar to English patterns like:

• Wearing sandals and shorts, the children run to the garden.
• While wearing sandals and shorts, the children are running to the garden.

Putting it first emphasizes the condition or appearance of the children before telling you the main action.

Yes. A natural alternative would be:

Watoto wanakimbia hadi bustanini wakiwa wamevaa sandaali na kaptula.

This still makes sense. The difference is mostly emphasis:

• starting with Wakiwa... gives the background first
• starting with Watoto wanakimbia... gives the main action first

Both are grammatical, but the original version sounds nicely descriptive and scene-setting.