По вечерам дети строят башню из кубиков, катают машинку по полу и собирают пирамидку снова.

По вечерам means in the evenings / every evening / in the evening time on a regular basis.

This is a very common Russian pattern:

• по утрам = in the mornings
• по вечерам = in the evenings
• по ночам = at night / nights
• по выходным = on weekends

Grammatically, по here is followed by the dative plural, so вечера becomes вечерам.

By contrast, вечером usually refers to in the evening more generally or on a particular occasion. In this sentence, the speaker is describing a routine, so по вечерам is the natural choice.

They are all direct objects, so they are in the accusative case.

The verbs are:

• строят what? → башню
• катают what? → машинку
• собирают what? → пирамидку

These nouns are feminine singular nouns ending in -я or -а in the dictionary form:

• башня → башню
• машинка → машинку
• пирамидка → пирамидку

So this is the normal feminine singular accusative pattern:

• -а → -у
• -я → -ю

Because из requires the genitive case.

Here из кубиков means out of blocks or from blocks. After из, Russian uses the genitive:

• кубик = a block
• кубики = blocks (nominative plural)
• кубиков = of blocks / from blocks (genitive plural)

So:

• башня из кубиков = a tower made of blocks

This is a very common pattern:

• дом из дерева = a house made of wood
• сок из яблок = juice from apples
• фигура из бумаги = a figure made of paper

Because по in the sense of movement along / over / across a surface usually takes the dative case.

So:

• пол = floor
• полу = dative singular

Катать машинку по полу means to roll/push a toy car along the floor.

This use of по is very common:

• идти по дороге = walk along the road
• ползти по земле = crawl on the ground
• водить пальцем по карте = move a finger over a map

The different endings come from different conjugation patterns.

• строить → строят
• катать → катают
• собирать → собирают

More specifically:

• строить is a second-conjugation verb, so the 3rd person plural ending is -ят
• катать and собирать are first-conjugation verbs, so the 3rd person plural ending is -ют

All three forms mean they do something:

• они строят = they build
• они катают = they roll
• они собирают = they assemble

Because Russian uses the present tense to describe habitual or repeated actions, just like English can.

So this sentence is not only about what the children are doing right now. It can also mean what they usually do in the evenings.

That is why строят, катают, and собирают work perfectly here. The phrase по вечерам already tells you this is a repeated activity.

The verbs here are imperfective:

• строить
• катать
• собирать

They are imperfective because the sentence describes:

• repeated actions
• a general routine
• the process of doing these things

That is exactly where imperfective verbs are normally used.

If you changed them to perfective forms, the meaning would shift toward completion or a single future result. For example, perfective present forms in Russian usually have future meaning, not ordinary present meaning.

So the imperfective forms are the natural choice here.

Because катать here is transitive: the children are moving something else, namely машинку.

So:

• катать машинку = to roll/push a toy car around
• кататься = to ride / to go for a ride / to enjoy moving oneself

Compare:

• Дети катают машинку. = The children roll a toy car.
• Дети катаются на велосипеде. = The children ride a bicycle.

So in this sentence, the children are not the ones riding. They are moving the toy.

Yes, both words are very natural child-related or small-object forms.

• машинка is often little car or toy car
• пирамидка often refers to the children’s stacking toy, not a giant pyramid

Russian uses diminutives very often when talking about:

• children
• toys
• small objects
• affectionate or everyday situations

So these forms sound especially natural in a sentence about children playing.

Yes. Дети is the subject for the entire series of actions:

• дети строят
• (дети) катают
• (дети) собирают

Russian does not need to repeat the subject when it stays the same. This is just like English:

• The children build a tower, roll a toy car on the floor, and assemble the stacking toy again.

Repeating дети before each verb would sound unnecessary unless you wanted special emphasis.

Снова means again.

In this sentence, it goes with собирают пирамидку:

• собирают пирамидку снова = assemble the stacking toy again

Yes, снова can often move, because Russian word order is flexible. For example:

• снова собирают пирамидку
• собирают снова пирамидку
• собирают пирамидку снова

These versions are all possible, but they may sound slightly different in emphasis. The version in your sentence puts снова at the end, which sounds natural and lets the sentence build step by step toward the final action.

Yes. Russian word order is more flexible than English word order.

The sentence as given is natural because it starts with the time phrase По вечерам, then names the subject дети, and then lists the actions.

But other orders are possible, for example:

• Дети по вечерам строят башню из кубиков...
• По вечерам дети снова собирают пирамидку...

Changing the order usually changes emphasis, not the basic meaning.

So the original version is not the only correct one, but it is a very normal and natural way to say it.