Маленький ребёнок закричал, когда увидел врача.

Because маленький has to agree with ребёнок.

Here, ребёнок is:

• masculine
• singular
• nominative (the subject of the sentence)

So the adjective is also masculine singular nominative: маленький.

Compare:

• маленький ребёнок = a little child
• маленькая девочка = a little girl
• маленького ребёнка = of a little child / a little child (in some cases)

Yes. Ребёнок is grammatically masculine, even though it can refer to either a boy or a girl.

That means words agreeing with it usually take masculine forms:

• маленький ребёнок
• ребёнок закричал

If you specifically want a feminine noun, you would use something like девочка.

Because закричал is perfective and usually means something like cried out, let out a scream, or started screaming.

• закричал = a single completed reaction / the moment the crying out began
• кричал = was shouting / was screaming / used to shout

In this sentence, the idea is a sudden reaction when the child saw the doctor, so закричал fits very well.

It shows past tense masculine singular.

In Russian past tense, verbs change for gender and number:

• закричал = masculine singular
• закричала = feminine singular
• закричало = neuter singular
• закричали = plural

Because ребёнок is grammatically masculine singular, the verb is закричал.

Because Russian often leaves out pronouns when the subject is clear from context.

So когда увидел врача literally looks like when saw the doctor, but Russian understands that it is the same subject as before: the child.

A natural English translation needs he, but Russian does not have to say он here.

Because the omitted subject of увидел is normally understood to be the same as the subject of the main clause: ребёнок.

Also, врача is in the accusative form, showing it is the object of увидел.

So the structure is:

• ребёнок = subject
• увидел = saw
• врача = the doctor / a doctor as the object

If you wanted when the doctor saw the child, you would say something like:

• когда врач увидел ребёнка

Because увидел is the perfective verb, meaning caught sight of or saw as a completed event.

• увидел = saw, caught sight of
• видел = saw / was seeing / had seen, with a more general or ongoing sense

Here the sentence focuses on a specific moment: the child saw the doctor, and then reacted. That is why увидел is natural.

Because врач is the direct object of увидел, so it must be in the accusative case.

For masculine animate nouns, the accusative singular looks like the genitive singular:

• nominative: врач
• accusative: врача

This is very common in Russian:

• Я вижу врача = I see the doctor
• Я знаю брата = I know my brother

But with masculine inanimate nouns, accusative usually looks like nominative:

• Я вижу стол = I see a table

It can mean either. Russian has no articles like a and the.

So увидел врача could mean:

• saw a doctor
• saw the doctor

The exact meaning depends on context.

Because когда увидел врача is a subordinate clause, and Russian normally separates subordinate clauses with a comma.

So:

• Маленький ребёнок закричал, когда увидел врача.

If the clause comes first, you still use a comma:

• Когда маленький ребёнок увидел врача, он закричал.

Yes. Russian word order is more flexible than English.

This sentence could also be:

• Когда маленький ребёнок увидел врача, он закричал.

Both are correct. The difference is mainly one of emphasis:

• Маленький ребёнок закричал, когда увидел врача starts with the main event: the child cried out.
• Когда маленький ребёнок увидел врача, он закричал starts with the time/background clause: when he saw the doctor.

Yes, you may also see ребенок in ordinary Russian writing. Native texts often omit the dots over ё and write е instead.

But the word is still pronounced ребёнок, not ребенок with a plain e sound.

For learners, writing ё is very helpful because:

• it shows the correct pronunciation
• it shows the stress
• it avoids confusion

The stress is:

Ма́ленький ребёнок закрича́л, когда́ уви́дел врача́.

A useful thing to remember: ё is always stressed.