Kisima hiki ni kirefu sana, kwa hiyo watoto wasikaribie.

In Swahili, demonstratives like this and that usually come after the noun.

So:

kisima hiki = this well

This is normal Swahili word order. English says this well, but Swahili says well this.

Because kisima belongs to the ki-/vi- noun class.

In Swahili, words like demonstratives must agree with the noun class of the noun they describe. Since kisima is singular in the ki-/vi- class, this becomes hiki.

A useful comparison:

• kisima hiki = this well
• visima hivi = these wells

So hiki is not random; it matches kisima.

For the same reason: adjective agreement.

The adjective stem is -refu, meaning deep/long/tall depending on context. Because kisima is in the ki-/vi- class, the adjective takes the class prefix ki-:

• -refu = deep/long/tall
• ki-refu = deep/long/tall for a class 7 noun

So:

kisima hiki ni kirefu = this well is deep

Swahili adjectives often change form to match the noun class of the noun they describe.

Ni is the copula here, similar to is/are in English.

So:

• Kisima hiki ni kirefu = This well is deep

It connects the subject kisima hiki with the description kirefu.

In Swahili, sana usually comes after the word it strengthens.

So:

• kirefu sana = very deep
• anafanya kazi sana = he/she works a lot
• nzuri sana = very good

That is why the sentence says kirefu sana, not sana kirefu.

Kwa hiyo means therefore, so, or for that reason.

It introduces the result or conclusion of the first idea:

• The well is very deep,
• therefore the children should not come near.

So kwa hiyo links cause and result.

This is a very common question.

• kwa sababu = because
• kwa hiyo = therefore / so

So:

• Watoto wasikaribie kwa sababu kisima hiki ni kirefu sana
  = The children should not come near because this well is very deep.

But in your sentence:

• Kisima hiki ni kirefu sana, kwa hiyo watoto wasikaribie
  = This well is very deep, therefore the children should not come near.

So kwa sababu introduces the reason, while kwa hiyo introduces the result.

The singular is mtoto = child.

So:

• mtoto = child
• watoto = children

This is part of the m-/wa- noun class for people. That noun class also affects verb agreement, which is why the verb begins with wa- in wasikaribie.

Wasikaribie can be broken down like this:

• wa- = they / class 2 subject marker
• -si- = negative marker
• karibie = subjunctive form of karibia = come near / approach

So the whole word means something like:

let them not come near
or more naturally in English,
they should not come near

This is a very common Swahili way to express a negative instruction for he/she/they.

Because the sentence is giving a warning or instruction, not stating a general fact.

• wasikaribie = they should not come near / let them not come near
• hawakaribii = they do not come near / they are not approaching

So:

• watoto wasikaribie = the children should not come near
• watoto hawakaribii = the children do not come near

The second one sounds like a description of what is happening, while the first one sounds like a prohibition or warning.

Yes, but it is a third-person command or prohibitive.

English does not have a simple one-word form for this, so we often translate it as:

• the children should not come near
• let the children not come near
• children must not come near

So it is not a plain statement. It is telling people what should or must not happen.

Here it means approach / come near.

So wasikaribie means they should not come near.

This can confuse learners because karibu is also a very common word meaning welcome or near. They are related in form, but in this sentence the verb karibia clearly means to approach / come close.

Yes, that is a very helpful literal understanding.

A fairly literal breakdown is:

• Kisima hiki = this well
• ni kirefu sana = is very deep
• kwa hiyo = therefore / so
• watoto wasikaribie = let the children not come near

That literal sense helps explain the grammar, even if a smoother English translation might be This well is very deep, so children should not come near.