Ребёнок осторожно перешёл через порог.

Ребёнок is a grammatically masculine noun in the singular. Because of that, the past-tense verb agrees with it in the masculine form: перешёл.

That does not automatically mean the child is male. Russian grammatical gender and real-life sex do not always match perfectly. Ребёнок can refer to any child unless the context tells you more.

If you want to specify sex, you would normally use:

• мальчик = boy
• девочка = girl

Осторожно is an adverb, meaning it describes how the action was done: the child crossed the threshold carefully / cautiously.

Compare:

• осторожный ребёнок = a careful/cautious child
  Here осторожный is an adjective describing the noun.
• ребёнок осторожно перешёл = the child crossed carefully
  Here осторожно is an adverb describing the verb.

Перешёл is the past tense, masculine singular form of the perfective verb перейти.

So it tells you:

• past: the action happened in the past
• masculine singular: it agrees with ребёнок
• perfective: the action is seen as completed

Other past forms would be:

• перешла = feminine
• перешло = neuter
• перешли = plural

This is a question of aspect, which is very important in Russian.

• перешёл is perfective: it presents the crossing as a completed whole
• переходил is imperfective: it presents the action as a process, repeated action, or background action

In this sentence, the speaker is talking about one completed event, so перешёл is the natural choice.

Compare:

• Ребёнок осторожно перешёл через порог.
  The child carefully crossed the threshold.
• Ребёнок осторожно переходил через порог.
  The child was crossing / used to cross the threshold carefully.
  This sounds more like description of a process or repeated action.

The prefix пере- often gives the idea of crossing from one side to another.

So:

• идти = to go, to walk
• перейти = to go across, cross over

In this sentence, перешёл suggests movement across the threshold, not just general walking.

Через is a preposition that normally takes the accusative case and often means across, through, or over, depending on context.

Here:

• через порог = across / over the threshold

Порог is in the accusative singular here. But because it is an inanimate masculine noun, its accusative singular looks the same as its nominative singular:

• nominative: порог
• accusative: порог

So the form does not change, even though the case does.

Russian через covers several English ideas, including through, across, and over, depending on the situation.

With порог it means crossing from one side of the threshold to the other, so in English that is often expressed as:

• across the threshold
• over the threshold

It does not mean going physically through the inside of something here. The basic idea is still crossing.

Yes. A very natural alternative would be:

• Ребёнок осторожно переступил порог.

This specifically suggests stepping over the threshold and is very idiomatic.

Your original sentence, Ребёнок осторожно перешёл через порог, is understandable and grammatically fine, but переступил порог is often a more typical choice when talking about a threshold.

Russian word order is fairly flexible, but the version here is a neutral, natural order:

• Ребёнок = subject
• осторожно = manner
• перешёл = verb
• через порог = direction/path

You can move parts around for emphasis, but the focus changes. For example:

• Осторожно ребёнок перешёл через порог
  Stronger emphasis on carefully
• Через порог ребёнок осторожно перешёл
  Stronger emphasis on the threshold

For a learner, the original order is a good standard pattern.

The letter ё is important here.

• ребёнок
• перешёл

In Russian, ё is always stressed. It is pronounced roughly like yo, though the exact sound depends on the surrounding consonants.

A useful thing to know: in many ordinary Russian texts, ё is often written as е instead. So you may sometimes see:

• ребенок
• перешел

But the pronunciation is still with ё, and for learners it is very helpful when the dots are written.

It can mean either, depending on context.

In this sentence, осторожно suggests that the child crossed the threshold with care, probably trying not to trip, fall, or make a mistake. So both carefully and cautiously are good ways to understand it.

Very often, the best English choice depends on style rather than grammar.