Wakiwa wamekula biskuti nyingi, watoto huanza kuomba maji.

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

In this sentence, it introduces a background/time clause:

• Wakiwa wamekula biskuti nyingi = when/after they have eaten many biscuits

So wakiwa helps set the situation in which the main action happens.

Because it is built from different pieces:

• wa- = subject marker for they (matching watoto)
• -ki- = a marker often used for when / if / while
• -wa = the verb be from kuwa

So:

• wakiwa = wa-ki-wa = when they are / while they are

That is why you see wa twice: the first one is the subject marker, and the second one is part of the verb kuwa.

Because wakiwa by itself only gives the idea when they are / being. It does not say in what state they are.

The next word gives that information:

• wamekula = they have eaten

So:

• wakiwa wamekula literally means when they are having eaten
• more natural English: when they have eaten or after they have eaten

This is a very common Swahili pattern:

• akiwa amechoka = when he is tired / when he has become tired
• wakiwa wamefika = when they have arrived

It breaks down like this:

• wa- = they
• -me- = perfect marker, often like have
• -kula = eat

So:

• wamekula = they have eaten

Here it shows that the eating happened before the children start asking for water.

Yes. In natural English, that is often the best way to understand it.

Depending on context, this part can be translated as:

• when they have eaten many biscuits
• after they have eaten many biscuits
• having eaten many biscuits

In this sentence, after is a very natural interpretation, because the eating comes first and then the asking for water begins.

huanza means usually begin, tend to begin, or begin in a habitual/general sense.

It comes from:

• hu- = habitual marker
• -anza = begin/start

So:

• watoto huanza kuomba maji = children usually begin to ask for water

This gives a general truth or repeated behavior.

By contrast:

• watoto wanaanza kuomba maji often sounds more like the children are beginning to ask for water in a specific situation, or just a more neutral present.

So huanza is a good choice if the sentence is describing what children generally do.

Yes, literally it is close to ask water, but in normal English we say ask for water.

In Swahili, the verb kuomba often takes directly the thing being requested:

• kuomba maji = to ask for water
• kuomba msaada = to ask for help
• kuomba pesa = to ask for money

So there is no extra word corresponding to English for here.

Here it is understood as plural because the meaning is many biscuits.

A useful thing to know is that some Swahili loanwords, including biskuti, often look the same in singular and plural. The form itself may not change.

So:

• biskuti can look the same whether you mean biscuit or biscuits
• the context tells you the meaning
• here nyingi and the overall meaning show that it is plural: many biscuits

Because adjectives in Swahili agree with the noun class of the noun they describe.

• -ingi = many / much
• with biskuti, it becomes nyingi

So:

• biskuti nyingi = many biscuits

Even though biskuti is a borrowed word, it behaves like a noun in the N-class pattern, where forms like nyingi are used.

Because watoto is a plural noun for people, and plural human nouns use the wa- agreement pattern.

So the sentence is internally matching:

• watoto = children
• wa- in wakiwa = they
• wa- in wamekula = they

This kind of agreement is a central part of Swahili grammar. The verb forms must match the noun class of the subject.

Because Swahili can begin with a time/background clause before giving the main subject and action.

So the structure is:

• Wakiwa wamekula biskuti nyingi = background/time situation
• watoto huanza kuomba maji = main statement

This is very natural, just like English:

• After eating many biscuits, the children start asking for water.

So the word order is not strange; it is just putting the time setting first.

Because Swahili does not have articles like English a, an, and the.

So:

• watoto can mean children or the children
• maji can mean water or the water

The exact meaning depends on context. In this sentence, English would usually translate it as the children and water, but Swahili does not need separate article words for that.