Дети играют рядом с горкой.

Because after рядом с (next to / beside), Russian uses the instrumental case.

• Dictionary form: горка
• Instrumental singular: горкой

So:

• с горкой = with / next to a slide
• рядом с горкой = next to the slide

This is one of the most important things to notice in the sentence: the preposition-like expression рядом с requires the noun after it to be in the instrumental.

Yes, с often means with, but in the expression рядом с, the two words work together as a set phrase meaning next to or beside.

So you should learn it as a unit:

• рядом с домом = next to the house
• рядом с машиной = next to the car
• рядом с горкой = next to the slide

Here, с is not being used in the basic English sense of with. It is part of the fixed expression рядом с.

It is instrumental singular feminine.

Here is the breakdown:

• горка = nominative singular
• горкой = instrumental singular

Why feminine?

Because горка is a feminine noun, and many feminine nouns ending in -а change to -ой / -ей in the instrumental singular.

Examples:

• мама → мамой
• девочка → девочкой
• горка → горкой

So горкой follows a very common pattern.

Играют is the 3rd person plural present-tense form of играть (to play).

The subject is дети (children), which is plural, so the verb must also be plural:

• ребёнок играет = the child is playing
• дети играют = the children are playing

So играют agrees with дети.

It can mean either, depending on context.

Russian present tense often covers both:

• Children play next to the slide
• The children are playing next to the slide

Russian does not usually make the same strict distinction English does between play and are playing in this kind of sentence. The context tells you whether it is habitual or happening right now.

So Дети играют рядом с горкой can describe:

• what they are doing now, or
• something they generally do

Because дети is the irregular plural used for children.

A learner often expects a regular plural, but Russian has some common irregular family/people words. Here:

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

So дети is a very important word to memorize as a basic vocabulary item.

It does not come from a normal plural pattern like adding a typical ending.

In practice, you usually connect it with ребёнок (child), even though the plural is irregular:

• ребёнок = child
• дети = children

You may also see дитя, which is an older/literary singular related historically to дети, but for everyday modern Russian, the normal singular-plural pair to learn is:

• ребёнок → дети

That is the pair most learners need.

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

The neutral order here is:

• Дети играют рядом с горкой.

But other orders are possible, for example:

• Рядом с горкой дети играют.
• Играют дети рядом с горкой.

These alternatives may sound more marked or emphasize different parts of the sentence. The original version is the most natural, straightforward way to say it in isolation.

So the basic idea is:

• English relies heavily on word order for grammar.
• Russian relies more on endings, and word order often affects emphasis rather than basic meaning.

Both are possible in Russian, depending on context.

Горка can mean:

• a small hill
• a playground slide

In the sentence Дети играют рядом с горкой, many learners would naturally picture a playground slide, because children commonly play near one. But the exact meaning depends on the situation or accompanying image.

So this word is a good example of a noun whose meaning is context-sensitive.

A natural stress pattern is:

• Де́ти игра́ют ря́дом с го́ркой.

Approximate pronunciation:

• ДЕ-ти и-ГРА-yut RYA-dəm s GOR-kəy

A few helpful notes:

• Дети is stressed on the first syllable: ДЕ́ти
• играют is stressed on гра́
• рядом is stressed on ря́
• горкой is stressed on го́р

Also, unstressed vowels in Russian are often reduced in pronunciation, so what you hear may sound less clear than the spelling suggests.

No, not in standard Russian.

The normal construction is:

• рядом с + instrumental

So you need:

• рядом с горкой
• рядом с домом
• рядом с детьми

Without с, the phrase would be ungrammatical in standard usage.

So it is best to memorize рядом с as a complete pattern.