Сначала дети хотели лепить снеговика поменьше, а потом снова стали лепить большого снеговика.

Because снеговика is the accusative singular form of an animate masculine noun.

In Russian, masculine animate nouns have:

• nominative: снеговик
• accusative: снеговика

Even though snowman is the direct object of лепить, Russian treats animate masculine direct objects with the same form as the genitive.

So:

• лепить снеговик = wrong here
• лепить снеговика = correct

For the same reason: the whole noun phrase is in the accusative singular, and the adjective has to agree with the noun.

Since снеговик is masculine singular animate, the adjective also takes the animate accusative form:

• большой снеговик = nominative, a big snowman
• лепить большого снеговика = accusative, to make a big snowman

So both words change:

• снеговик → снеговика
• большой → большого

Поменьше means something like smaller or a bit smaller, not simply small.

That is the key difference:

• маленький снеговик = a small snowman
• снеговик поменьше = a smaller snowman / a somewhat smaller one

So the sentence is comparing the size to some other possible size. The children did not just want a small snowman; they wanted a smaller one.

Because поменьше is not behaving like a regular adjective such as большой or маленький. It is an indeclinable comparative form.

That means it does not change for:

• case
• gender
• number

So you get:

• дом побольше
• книгу поинтереснее
• снеговика поменьше

By contrast, regular adjectives do decline:

• большой снеговик
• большого снеговика

It usually softens the comparison and makes it sound more natural in everyday speech.

So:

• меньше = smaller / less
• поменьше = a bit smaller, somewhat smaller, on the smaller side

In many contexts, поменьше sounds less blunt and more conversational.

Here it suggests:

• not necessarily much smaller
• just somewhat smaller than the larger version they had in mind

Because лепить is imperfective, and that fits the situation well.

Imperfective is natural here because the sentence talks about:

• intention: хотели лепить
• beginning/resuming an activity: стали лепить
• the process itself, not the finished result

If you used слепить instead, the focus would be more on completing one snowman.

Compare:

• хотели лепить снеговика = they wanted to be making a snowman
• хотели слепить снеговика = they wanted to make/finish a snowman

Both can be possible in some contexts, but лепить fits very naturally with the idea of the activity in progress.

Because both verbs are followed by another verb in the infinitive.

This is very common in Russian:

• хотеть + infinitive = to want to do something
• стать + infinitive = to begin/start doing something

So:

• хотели лепить = wanted to make
• стали лепить = began to make / started making

Also, Russian does not use a separate word like English to before the infinitive. The infinitive itself already has that meaning.

It means began making or started making.

Literally, стали is the past plural of стать, and with an infinitive it often means to start doing something.

So:

• стали лепить = started making
• снова стали лепить = started making again / went back to making

In this sentence, it suggests a change of plan or a return to an earlier idea.

Because а often marks a contrast or a change of direction.

Here the sentence is structured as:

• first they wanted one thing
• then they did something different

So а works well because it sets up that contrast:

• Сначала... а потом... = first..., but then... / first..., and then...

In English, this may be translated with and then or but then, depending on style. Russian а often sits somewhere between simple and and contrastive but.

They do different jobs:

• потом = then, afterwards, later
• снова = again

So together they mean something like:

• and then again
• and later they went back to
• and then once more

This tells you not just that the second action happened later, but that it was in some sense a repeat or a return.

Because the subject is дети, which is plural.

In the Russian past tense, verbs agree with the subject in:

• number
• and in the singular, also gender

So:

• ребёнок хотел = one child wanted
• дети хотели = the children wanted

And:

• ребёнок стал = one child started
• дети стали = the children started

Since дети is plural, both verbs must be plural.

Russian word order is fairly flexible, but the version in the sentence is very natural.

This order works well because it presents the timeline clearly:

• Сначала = first
• а потом = and then / but then
• снова = again

You could also hear variants such as:

• Дети сначала хотели лепить снеговика поменьше...
• А потом дети снова стали лепить большого снеговика.

These are still grammatical, but they shift the emphasis slightly.

The original sentence sounds smooth and neutral:

• first the time frame
• then the subject
• then what happened

Usually it is better to keep it here.

Russian often repeats a noun where English might prefer to leave it out, especially when:

• the description changes
• clarity matters
• the speaker wants a neat contrast

Here the contrast is:

• снеговика поменьше
• большого снеговика

Repeating снеговика makes that contrast very clear and natural. Without the noun, the sentence would sound less complete or more dependent on context.