Meine Enkelin und ihre Freundin spielen auf der Wippe.

Because Enkelin is a feminine noun.

In German, possessive words like mein change their ending to match the noun’s gender, number, and case.
Here, Enkelin is:

• feminine
• singular
• nominative (because it is part of the subject)

So mein becomes meine.

Compare:

• mein Enkel = my grandson
• meine Enkelin = my granddaughter

For the same reason: Freundin is also a feminine singular noun.

So the possessive word ihr- takes the ending -e here:

• ihre Freundin = her friend / their friend

In this sentence, the most natural meaning is her friend.

A useful note: ihre can mean different things depending on context, including:

• her
• their
• its (in some contexts)
• formal your

But in this sentence, learners will normally understand it as her.

Because the subject is plural.

The subject is:

• Meine Enkelin und ihre Freundin

That means my granddaughter and her friend — two people.

So the verb must be in the 3rd person plural:

• sie spielen = they play

Compare:

• Meine Enkelin spielt. = My granddaughter plays.
• Meine Enkelin und ihre Freundin spielen. = My granddaughter and her friend play.

Because auf is a two-way preposition.

With two-way prepositions, German uses:

• accusative for movement toward a destination
• dative for location

Here, the sentence describes where they are playing, not movement onto the seesaw. So German uses the dative:

• auf der Wippe = on the seesaw

Compare:

• Sie klettern auf die Wippe. = They climb onto the seesaw.
  (movement → accusative)
• Sie spielen auf der Wippe. = They are playing on the seesaw.
  (location → dative)

Since die Wippe is feminine, its dative form is der Wippe.

Because der is not always masculine. It depends on the case.

The noun is:

• die Wippe in the nominative
• der Wippe in the dative

So here:

• auf der Wippe

means on the seesaw, with der showing dative feminine.

A quick article chart for die Wippe:

• nominative: die Wippe
• accusative: die Wippe
• dative: der Wippe
• genitive: der Wippe

The basic structure is:

• Meine Enkelin und ihre Freundin = subject
• spielen = verb
• auf der Wippe = prepositional phrase

So the sentence is:

Subject + Verb + Other information

This is a standard German main clause, where the finite verb is in second position.

Even though the subject contains several words, it still counts as one unit before the verb:

• Meine Enkelin und ihre Freundin | spielen | auf der Wippe

Yes. German often allows different word orders as long as the verb stays in second position in a main clause.

For example:

• Auf der Wippe spielen meine Enkelin und ihre Freundin.

This is also grammatical. It puts more emphasis on where they are playing.

So both work:

• Meine Enkelin und ihre Freundin spielen auf der Wippe.
• Auf der Wippe spielen meine Enkelin und ihre Freundin.

The first one is more neutral.

It can mean either, depending on context.

• eine Freundin can mean a female friend
• meine Freundin can also mean my girlfriend

In this sentence, ihre Freundin will usually be understood as her friend, especially in a simple family-context sentence like this one.

German often relies on context here, just as English sometimes does.

Enkel is the masculine form, and Enkelin is the feminine form.

• der Enkel = grandson
• die Enkelin = granddaughter

German often forms feminine nouns for people by adding -in:

• Freund → Freundin
• Lehrer → Lehrerin
• Enkel → Enkelin

The plural of Enkelin is:

• die Enkelinnen

Because all nouns are capitalized in German.

So in this sentence:

• Enkelin
• Freundin
• Wippe

are capitalized because they are nouns.

This is one of the most noticeable spelling rules in German.

Because und is simply joining two parts of the subject:

• meine Enkelin
• ihre Freundin

German does not use a comma before und in a simple coordination like this.

So:

• Meine Enkelin und ihre Freundin ...

is correct without a comma.

It is literally to play, but in German, just like in English, it can cover several related ideas.

Here, spielen auf der Wippe means they are playing on the seesaw, which may imply:

• enjoying themselves
• being active
• using the playground equipment

So spielen is the normal, natural verb here.

The preposition auf usually gives the idea of on, onto, or sometimes at depending on context. With something like Wippe, the natural interpretation is physically on it.

So:

• auf der Wippe = on the seesaw

If German wanted to say something more like next to the seesaw, it would use a different preposition, for example:

• neben der Wippe = next to the seesaw

So auf strongly suggests that they are on the seesaw itself.