A criança caiu da cadeira.

Because criança is a grammatically feminine noun in Portuguese, so it takes a.

This is true even if the child is a boy. In other words, grammatical gender and biological sex are not always the same thing.

• a criança = the child
• uma criança = a child

If you want to make the sex clear, you would usually use:

• o menino = the boy
• a menina = the girl

Caiu means fell.

It is the 3rd person singular form of the verb cair in the pretérito perfeito (simple past / preterite).

• cair = to fall
• eu caí = I fell
• ele/ela caiu = he/she fell

So A criança caiu means The child fell.

Because de + a normally contracts into da in Portuguese.

So:

• de + a = da
• de + o = do
• de + as = das
• de + os = dos

That means:

• da cadeira = from the chair / off the chair

Using de a cadeira would sound unnatural in normal Portuguese.

It can mean either, depending on how you translate it naturally into English.

In this sentence, A criança caiu da cadeira, the most natural English translation is often:

• The child fell off the chair.

Literally, da cadeira is from the chair, but English usually says fell off the chair in this situation.

Portuguese often uses articles where English may not focus on them as much.

Here, da cadeira means from the chair or off the chair, referring to a specific chair in the situation.

• da cadeira = from/off the chair
• de uma cadeira = from/off a chair

If you said A criança caiu de uma cadeira, it would sound less specific: the child fell off a chair, not necessarily a chair already known in the context.

This is the normal basic word order in Portuguese:

subject + verb + complement

So:

• A criança = subject
• caiu = verb
• da cadeira = prepositional phrase showing where/from where

This is the most neutral, standard order.

In Brazilian Portuguese, criança is pronounced approximately like:

kree-AHN-sah

A few important points:

• cri sounds like kree
• ã is nasal, so the sound comes through the nose
• ç sounds like s

So the word has a nasal sound in the middle: cri-AN-ça.

In Brazilian Portuguese, caiu is roughly:

kah-YOO

More exactly, it has two syllables:

• ca
• iu

So it is not pronounced like one flat syllable. The stress is on the second part: ca-IU.

Because Portuguese, like English, does not need a subject pronoun when the noun itself is already there.

So:

• A criança caiu. = The child fell.
• Ela caiu. = She fell.

Once you say a criança, you do not need to add ela.

This is a tense difference.

• caiu = fell / has fallen in a completed event
• caía = was falling or used to fall

So:

• A criança caiu da cadeira. = The child fell off the chair.
  -> one completed event

• A criança caía da cadeira...
  -> this sounds incomplete by itself, and usually needs more context, like
  A criança caía da cadeira quando a mãe chegou.
  = The child was falling off the chair when the mother arrived.

Yes.

Criança is a general word for child and can also correspond to kid in many contexts.

Depending on tone and context, a translation could be:

• The child fell off the chair.
• The kid fell off the chair.

Child is more neutral; kid is more informal.

Yes, it is completely natural.

A criança caiu da cadeira. is a normal, everyday sentence in Brazilian Portuguese. It sounds like something a native speaker would naturally say.