Daktari alisema ati mtoto asipokunywa maji ya kutosha, kuhara kutamchosha zaidi.

Here ati introduces reported speech, so alisema ati... means said that...

Yes, kwamba could also be used in many contexts:

• Daktari alisema kwamba...
• Daktari alisema ati...

A learner should know that ati is often more conversational and common in speech. In some situations it can also sound like claiming that or saying that, but in this sentence it simply introduces what the doctor said.

Asipokunywa is one verb word made of several parts:

• a- = he/she
• -si- = negative
• -po- = conditional marker, often giving the sense of if/when
• kunywa = drink

So:

a-si-po-kunywa
= if he/she does not drink

This is a very normal Swahili pattern: instead of using a separate word for if not, Swahili often builds the meaning directly into the verb.

Because Swahili normally uses both:

• the full noun subject
• and the subject marker inside the verb

So mtoto asipokunywa is perfectly normal.
Literally, it is something like:

• the child
• he/she-if-not-drink

To an English speaker this can feel repetitive, but in Swahili it is standard agreement, not unnecessary repetition.

Maji ya kutosha means enough water or sufficient water.

Breakdown:

• maji = water
• ya = agreement word matching maji
• kutosha = to be enough

So literally it is something like water of being enough, but natural English is enough water.

The reason it is ya is that maji belongs to noun class 6, and words connected to it must agree with that class.

Here kuhara functions as a noun meaning diarrhea.

So:

• kuhara kutamchosha zaidi = diarrhea will tire him/her more

A learner may notice that kuhara looks like an infinitive verb form, and that is true historically in shape. But in this sentence it is being used as the subject of the clause, so it behaves like a noun.

Kutamchosha can be broken down like this:

• ku- = subject marker agreeing with kuhara
• -ta- = future tense
• -m- = object marker meaning him/her
• -chosha = tire, exhaust

So:

ku-ta-m-chosha
= it will tire him/her

In full context, it refers to kuhara.

The -m- refers back to mtoto.

In the second clause, kuhara is the subject, not the child. The child is the one affected by the action, so the child becomes the object:

• kuhara = subject
• -m- = the child, as object
• -chosha = tire

So the structure is basically:

diarrhea will tire the child more

The future tense -ta- shows the expected result of the condition.

The sentence has this logic:

• if the child does not drink enough water
• diarrhea will tire him/her more

So the future marks the consequence that will happen after or as a result of that condition.

Zaidi means more, further, or even more.

Here it means that the diarrhea will make the child more tired than before, or even more exhausted.

So:

• kutamchosha = will tire him/her
• kutamchosha zaidi = will tire him/her more / even more

No. Swahili does not mark gender here.

• mtoto just means child
• a- can mean he or she
• -m- can mean him or her

So the sentence could refer to either a boy or a girl. The meaning stays the same.