Learn Hardcore German: Die Kleinen im Kindergarten mögen es, wenn die Verantwortliche mit ihnen im Sand spielt. - The little ones in kindergarten like it when the woman in charge plays with them in the sand.

In die Kleinen, Kleinen is originally an adjective (klein = small, little), but here it is used as a noun meaning “the little ones” (i.e., the small children).

In German, whenever an adjective is used as a noun, it is capitalized and declined like an adjective. So:

die Kleinen = the little ones (plural, any gender, usually children)
der Kleine = the little one (singular, masculine)
die Kleine = the little one (singular, feminine)

That’s why Kleinen is capitalized: it is a nominalized adjective being used as a noun.

Why does the sentence use die Kleinen instead of just die Kinder?

Both are possible, but they have slightly different nuances:

die Kinder im Kindergarten = the children in the kindergarten
→ neutral, just states that they are children.
die Kleinen im Kindergarten = the little ones in the kindergarten
→ more affectionate or descriptive; emphasizes that they are small, young kids.

So die Kleinen sounds a bit more warm, informal, or caring, like how English speakers might say “the little ones” when talking about small children.

What is im Kindergarten exactly, and why is it written like that instead of in dem Kindergarten?

im is a contraction of:

in + dem → im

Kindergarten is masculine: der Kindergarten.
The preposition in can trigger the dative case when it indicates location (“in” as in “inside somewhere”).

So:

in dem Kindergarten = in the kindergarten (dative singular masculine)
In spoken and standard written German this contracts to im Kindergarten.

This kind of contraction is very common:

in dem → im
an dem → am
zu dem → zum
bei dem → beim

Why do we say mögen es, wenn ...? Can’t we just say mögen, wenn ...?

With verbs like mögen, lieben, hassen etc., when the object is a subordinate clause (like a wenn-clause), German very often uses a dummy object “es”:

Sie mögen es, wenn … = They like it when …
Ich hasse es, wenn … = I hate it when …
Wir lieben es, wenn … = We love it when …

Grammatically, es is the direct object of mögen, and the wenn-clause explains what kind of thing they like.

Without es, Sie mögen, wenn … is possible but sounds unusual or incomplete in modern standard German. Native speakers would nearly always say Sie mögen es, wenn … in this structure.

What does die Verantwortliche mean, and why is it capitalized?

Verantwortliche comes from the adjective verantwortlich = responsible.
Here it is turned into a noun: die Verantwortliche = the person in charge / the responsible person (here, clearly the female person in charge of the children, like the teacher or caregiver).

Reasons for capitalization:

It is used as a noun (a person) → all nouns and nominalized adjectives in German are capitalized.
The article die marks it as a feminine singular noun:
- die Verantwortliche = the (female) person responsible
- der Verantwortliche = the (male) person responsible
- die Verantwortlichen = the responsible people (plural)

So Verantwortliche is another example of a nominalized adjective (like Kleinen).

How do we know die Verantwortliche is singular and not plural, since die can also be plural?

You’re right that die can mean either:

feminine singular → die Frau (the woman)
plural (all genders) → die Frauen (the women)

We know die Verantwortliche here is feminine singular because of the verb form and context:

… wenn die Verantwortliche … spielt.
→ spielt is 3rd person singular.

If it were plural, the verb would be spielen:

… wenn die Verantwortlichen mit ihnen im Sand spielen.
→ “when the people in charge play with them in the sand.”

So the singular verb spielt signals that die Verantwortliche is one female person.

Why is Verantwortliche feminine here? Could it also refer to a man?

Yes, the underlying adjective verantwortlich itself is gender-neutral. The article and ending decide the gender/number:

die Verantwortliche = the responsible person (female, singular)
der Verantwortliche = the responsible person (male, singular)
die Verantwortlichen = the responsible people (plural)

In this sentence, die Verantwortliche clearly refers to a female person in charge, likely a female kindergarten teacher or caregiver. If the person in charge were male, you would say:

… wenn der Verantwortliche mit ihnen im Sand spielt.

Why is there a comma before wenn?

In German, subordinate clauses (Nebensätze) are usually separated from the main clause with a comma.

Here:

Main clause: Die Kleinen im Kindergarten mögen es
Subordinate clause: wenn die Verantwortliche mit ihnen im Sand spielt.

Because wenn introduces a subordinate clause, you must put a comma before wenn:

…, wenn die Verantwortliche mit ihnen im Sand spielt.

This is a fixed rule in standard written German.

What exactly does wenn mean here, and why not als?

wenn and als can both mean “when”, but they are used differently:

wenn is used for:
- repeated events in the past, present, or future
- general conditions (“whenever / if”)
als is used for:
- a single, specific event in the past

In this sentence:

Die Kleinen … mögen es, wenn die Verantwortliche mit ihnen im Sand spielt.
→ This describes something that happens repeatedly or in general:
“they like it whenever she plays in the sand with them”.

So wenn is correct.
Als would sound wrong here, because the sentence is not about one specific past occasion.

Why is the verb spielt at the end of the wenn-clause?

German word order rule:

In a main clause, the conjugated verb is in second position.
In a subordinate clause introduced by a conjunction like wenn, the conjugated verb goes to the very end of the clause.

So:

Main clause: Die Kleinen … mögen es
→ mögen in 2nd position.
Subordinate clause: wenn die Verantwortliche mit ihnen im Sand spielt
→ spielt is the conjugated verb → placed at the end.

This is why you get:

…, wenn die Verantwortliche mit ihnen im Sand spielt.
and not
…, wenn die Verantwortliche spielt mit ihnen im Sand. (word-for-word English order, incorrect in German here).

Why is it mit ihnen and not mit sie?

The preposition mit always takes the dative case in German.

The personal pronoun sie (they/them) changes in the dative:

Nominative: sie (they)
Accusative: sie (them – direct object)
Dative: ihnen (to them / with them)

Since mit requires dative:

mit + ihnen = with them

So mit sie is wrong; it must be mit ihnen.

Why is it im Sand? Does it mean “in the sand” or “on the sand”?

Literally, im Sand = in dem Sand → “in the sand.”

In practice, German im Sand spielen is used for what English usually calls “playing in the sand” (digging in it, building sandcastles, etc.). In English you might say “in the sand” or “in the sandbox” depending on context.

German could also say im Sandkasten (“in the sandbox”), but im Sand is perfectly natural, especially when it’s clear from the context that we’re talking about a sandbox or a sandy play area.

Could we say Die Kinder mögen es, wenn die Verantwortliche mit ihnen im Sand spielt instead? Is that different?

Yes, you can say:

Die Kinder mögen es, wenn die Verantwortliche mit ihnen im Sand spielt.

This is grammatically correct and means essentially the same thing.

Difference in nuance:

Die Kinder – neutral, factual: the children.
Die Kleinen – gives a slightly more affectionate / informal tone: the little ones.

So the sentence with die Kleinen sounds a bit more child-centered and warm, but both versions are correct.

Could we use gern instead of mögen es, wenn …? For example: Die Kleinen im Kindergarten spielen gern mit der Verantwortlichen im Sand. Is that equivalent?

Yes, you can rephrase the idea using gern:

Die Kleinen im Kindergarten spielen gern mit der Verantwortlichen im Sand.
= The little ones in kindergarten like playing in the sand with the person in charge.

Differences:

mögen es, wenn … focuses on liking a situation or event:
- Sie mögen es, wenn … = They like it when …
gern + Verb focuses more on liking to do an activity:
- Sie spielen gern … = They like to play …

So your alternative sentence is natural German; it just frames the preference in a slightly different way (emphasizing their activity rather than the situation when she joins).