Дети любят плескаться у берега, а мой сын уже учится нырять с пирса.

Because after verbs like любить meaning to like / to love doing something, Russian normally uses an infinitive for the second action:

• Дети любят плескаться = Children like to splash around
• compare: Я люблю читать = I like reading / I like to read

So only любят changes to match the subject дети, while плескаться stays in the infinitive.

Плескаться means something like to splash around, especially in water in a playful way.

The ending -ся does not always mean a literal reflexive idea like oneself. Very often it makes the verb more intransitive or gives it a middle/reflexive sense.

Compare:

• плескать воду = to splash water
• плескаться в воде = to splash around in the water

So here плескаться is the natural verb for children playing in shallow water.

У берега uses the preposition у, which takes the genitive case. So:

• dictionary form: берег
• genitive singular: берега

Here у берега means by the shore / near the shore.

This is different from на берегу:

• у берега = near the shore, often still in or right next to the water
• на берегу = on the shore, on land

So Дети любят плескаться у берега gives the image of children playing in the water close to the shoreline.

The comma is needed because а is connecting two separate clauses:

• Дети любят плескаться у берега
• мой сын уже учится нырять с пирса

In Russian, when а joins two independent clauses, you normally put a comma before it.

As for meaning, а often shows a contrast or switch of focus. It is not always as strong as but, and not always as neutral as and. Here it feels like:

• Children like splashing near the shore, while my son is already learning to dive from the pier

So а works well because the sentence shifts from a general statement about children to a more specific statement about my son.

Because учится is a conjugated verb form: third person singular, present tense.

• учиться = to learn / to study — infinitive
• учится = he/she learns, is learning — finite verb

Here the subject is мой сын, so we need the form учится:

• мой сын учится = my son is learning

A common Russian spelling rule is:

• что делать? → infinitive → usually with -ться
• что делает? → finite verb → usually with -тся

So:

• учиться = what to do?
• учится = what is he doing?

Russian commonly uses учиться + infinitive to mean to learn to do something.

So:

• учится нырять = is learning to dive

This is a very standard pattern:

• учиться плавать = to learn to swim
• учиться водить = to learn to drive
• учиться говорить по-русски = to learn to speak Russian

If the sentence said мой сын уже ныряет, that would mean my son is already diving, not learning to dive.

Because учиться usually takes an infinitive that names a general skill or ongoing activity, and that is normally the imperfective verb.

• нырять = imperfective, to dive in general / to be diving / to dive repeatedly
• нырнуть = perfective, to make one dive / to dive once

When someone is learning a skill, Russian usually focuses on the activity as a process:

• учиться нырять = to learn how to dive

Using нырнуть here would sound unnatural in most contexts, because it points to one completed dive rather than the general ability.

Уже means already.

Here it suggests that the son has reached a new stage:

• мой сын уже учится нырять = my son is already learning to dive

It can imply:

• this is happening by now
• maybe earlier he was too young or not ready
• there is a mild sense of progress or development

So уже gives the sentence a natural feeling of he’s advanced enough now.

Here с means from / off a place, so it takes the genitive case:

• dictionary form: пирс
• genitive singular: пирса

So:

• с пирса = from the pier / off the pier

Compare:

• на пирсе = on the pier
• с пирса = off the pier / from the pier

This is a very common pattern in Russian:

• с моста = from the bridge
• с крыши = from the roof
• со стола = from the table

The word order can be changed, because Russian word order is fairly flexible. But the version in the sentence is the most neutral and natural.

Current order:

• Дети любят плескаться у берега, а мой сын уже учится нырять с пирса.

This sounds smooth and unmarked.

You could move things for emphasis, for example:

• С пирса мой сын уже учится нырять — emphasizes from the pier
• Мой сын уже с пирса учится нырять — more conversational, but more marked

So the sentence as given is probably the best basic model for a learner, even though other orders are possible.

Yes, but it is worth noticing that it is an irregular plural.

• singular: ребёнок = child
• plural: дети = children

So Russian does not form this plural in a regular way. Learners simply have to memorize it.

In this sentence, дети is in the nominative plural, because it is the subject of любят.