Летом дети любят плескаться в тёплой воде.

Летом is the instrumental singular form of лето and is often used adverbially to mean in summer or during the summer.

This is very common with the seasons:

• зимой = in winter
• весной = in spring
• летом = in summer
• осенью = in autumn

So Летом дети любят... means In summer, children like...

Russian often expresses time this way without a preposition.

Дети means children and is plural.

Its singular is ребёнок = child.

This is an irregular pair, so you cannot guess дети from ребёнок very easily. It is just something to learn as a set:

• ребёнок = child
• дети = children

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

After любить = to love / to like, Russian often uses an infinitive to say that someone likes doing something.

So:

• дети любят плескаться = children like to splash around

This is similar to English like to do something.
English also often uses a gerund, as in like splashing around, but Russian normally uses the infinitive here.

Also, любят is the 3rd person plural form of любить, agreeing with дети:

• я люблю
• ты любишь
• он / она любит
• мы любим
• вы любите
• они любят

Плескаться means something like:

• to splash around
• to frolic in the water
• to play in the water

It suggests playful movement in water, especially by children.

So this sentence is not just about making one splash. It gives the image of children happily playing and splashing in warm water.

The ending -ся marks a reflexive verb.

Very often, this makes a verb more intransitive or gives it a meaning like do something oneself / be engaged in something.

Compare:

• плескать = to splash something / to make splashes
• плескаться = to splash around, to splash about in the water

Here, плескаться is the natural verb because the children are not splashing some object; they are themselves in the water, moving and splashing around.

So -ся is an important part of the meaning.

Because after в meaning in, when you are talking about location, Russian uses the prepositional case.

So:

• вода = water
• в воде = in water

And the adjective must match:

• тёплая вода = warm water
• в тёплой воде = in warm water

So both words change:

• тёплая → тёплой
• вода → воде

A very useful contrast is:

• в тёплой воде = in warm water, inside it, location
• в тёплую воду = into warm water, direction/motion

In this sentence, it is prepositional.

You are right that the form воде can look like dative as well, because for many feminine nouns the dative singular and prepositional singular are identical in form.

Here the meaning and the preposition tell you the case:

• в воде = in the water → prepositional, because it expresses location
• к воде = toward the water → dative, because к requires the dative

So in в тёплой воде, the function is clearly locational, so we analyze it as prepositional.

Ё is pronounced like yo and is always stressed.

So тёплой sounds roughly like TYOP-lay.

A few important things:

• ё always carries the stress
• in normal Russian writing, ё is often replaced by е
• so you may sometimes see теплой воде, but it is still pronounced тёплой воде

That is very common in printed Russian, and learners need to get used to recognizing it.

No, Russian word order is flexible.

The sentence Летом дети любят плескаться в тёплой воде is natural because it starts with the time expression летом, setting the scene first:

• In summer, children like splashing around in warm water

You could also say:

• Дети любят плескаться в тёплой воде летом
• Дети летом любят плескаться в тёплой воде

These are all possible, but they shift the emphasis slightly.

The given version is a very normal way to present the general time background first.

Russian has no articles.

So Russian nouns do not have separate words corresponding to a, an, or the.

That means дети can mean different things depending on context, such as:

• children
• the children
• sometimes some children

In this sentence, it is understood generically: children in general.

That is why Russian can say simply дети любят... with no article.

Yes, aspect matters.

Плескаться is imperfective, and that fits the sentence because the meaning is general and habitual:

• children like doing this
• it is a repeated or typical activity
• no single completed action is being emphasized

Russian normally uses the imperfective in this kind of statement.

A perfective verb would usually suggest a single completed event or a bounded action, which does not fit as well with a general statement about what children like.

So любят плескаться is the natural choice for like splashing around in general.