Se a papinha ficar fria, eu vou esquentar de novo antes do próximo cochilo.

Because after se when you are talking about a possible future situation, Portuguese normally uses the future subjunctive.

• ficar here = future subjunctive
• fica = present indicative

So:

• Se a papinha ficar fria... = if the baby food gets/is cold
• Se a papinha fica fria... sounds wrong for this kind of future condition in standard Portuguese.

This is a very common pattern:

• Se chover, eu fico em casa.
• Se ele chegar cedo, nós saímos.
• Se você precisar, me liga.

English does not have a special form here, but Portuguese does.

Ficar often means to become or to end up in a state.

So:

• ficar fria = to become cold / to get cold

That is why it works well in this sentence: the baby food is not cold yet, but it may become cold.

Compare:

• A papinha está fria. = The baby food is cold.
  (describes its current state)
• A papinha ficou fria. = The baby food got cold / became cold.
• Se a papinha ficar fria... = If the baby food gets cold...

Papinha is a diminutive form, usually from papa or related baby-food wording. In Brazilian Portuguese, papinha commonly refers to baby food, baby mush, or puréed food for a baby.

The ending -inha is a diminutive suffix. It can suggest:

• small size
• affection
• something associated with babies or cuteness

So papinha sounds natural and caring in a context involving a baby.

Portuguese uses definite articles much more often than English.

So a papinha literally looks like the baby food, but in context it may simply refer to the baby food already known in the situation.

This is very normal in Portuguese:

• O bebê dormiu.
• A comida esfriou.
• A mamadeira está na cozinha.

English often omits the where Portuguese prefers an article.

In Portuguese, adjectives often come after the noun.

So:

• papinha fria = cold baby food

This is the most neutral order. Sometimes adjectives can come before the noun, but that often changes the tone or emphasis.

Basic pattern:

• casa grande = big house
• água fria = cold water
• comida quente = hot food

Yes, you absolutely could say just vou esquentar.

Portuguese often drops subject pronouns when they are clear from the verb form. So both are natural:

• Eu vou esquentar de novo...
• Vou esquentar de novo...

Using eu adds a bit of emphasis, clarity, or contrast. For example, it can sound like:

• I’ll heat it up again
• I’m the one who’ll heat it up again

So the pronoun is optional here, not required.

In everyday Brazilian Portuguese, the periphrastic future with ir + infinitive is much more common in speech.

So:

• vou esquentar = I’m going to heat it up / I will heat it up

This sounds natural and conversational.

The simple future is grammatically correct:

• esquentarei

But it sounds more formal, literary, or less common in casual speech.

In normal spoken Brazilian Portuguese, people usually prefer:

• vou fazer
• vou ligar
• vou sair
• vou esquentar

It can mean either, depending on context.

• esquentar = to heat, to warm, to warm up

In this sentence, esquentar de novo usually means warm it up again or reheat it.

A very close alternative would be requentar, which specifically means to reheat. But esquentar de novo is extremely natural and common.

Yes, here de novo means again.

• esquentar de novo = heat up again

It is very common in spoken Brazilian Portuguese.

Novamente also means again, but it can sound a bit more formal or careful depending on context.

Compare:

• Vou tentar de novo. = I’ll try again.
• Vou tentar novamente. = I’ll try again.
  (slightly more formal or polished)

In casual speech, de novo is often the more natural choice.

Because antes de is the correct structure here.

You need the preposition de after antes when it is followed by a noun:

• antes de dormir
• antes da aula
• antes do almoço
• antes do próximo cochilo

Also, do = de + o.

So:

• antes do próximo cochilo = before the next nap

This contraction is required:

• de + o = do
• de + a = da

Not exactly.

• cochilo = nap
• sono = sleep / sleepiness

So:

• o próximo cochilo = the next nap
• estou com sono = I am sleepy
• o bebê tirou um cochilo = the baby took a nap

In this sentence, cochilo is specific because it refers to a nap as an event or period of sleep.

Some adjectives commonly go before the noun in Portuguese, and próximo is very often one of them when it means next.

So:

• o próximo cochilo = the next nap

That word order is the most natural here.

You will see similar patterns with:

• na próxima semana = next week
• no próximo mês = next month
• na próxima vez = next time

Not in the same place with the same meaning.

• fria = cold
• quentinha = nice and warm / warmish / pleasantly warm

If you said:

• Se a papinha ficar quentinha...

that would mean If the baby food gets nice and warm..., which changes the meaning completely.

Also, quentinha has the diminutive -inha, which often adds a cozy or affectionate tone. It is common in everyday speech, but it does not mean cold or not too cold. It still means warm.

Yes, it sounds very natural.

The vocabulary and structure fit everyday Brazilian Portuguese well:

• papinha is natural for baby food
• ficar fria is the normal way to say get cold
• vou esquentar de novo is very common spoken Portuguese
• próximo cochilo fits naturally in a routine involving a baby

A Brazilian speaker might also say slight variations such as:

• Se a papinha esfriar, eu vou esquentar de novo antes do próximo cochilo.
• Se a papinha ficar fria, esquento de novo antes do próximo cochilo.

But the original sentence is fully natural.