Der Hund lässt sich von allen Kindern streicheln.

This is a very common German pattern:

sich lassen + infinitive

It often means can be ...-ed or allows itself to be ...-ed.

So:

• Der Hund lässt sich streicheln = The dog lets itself be petted
• More naturally in English: The dog allows people to pet it / The dog can be petted

In this sentence, the idea is that the dog is willing and approachable, not resisting.

sich is the reflexive pronoun for third person singular and plural.

Here the subject is der Hund, so the dog is both:

• the thing doing the allowing
• the thing receiving the petting

That is why German uses sich.

Compare:

• Der Hund lässt sich streicheln. = The dog lets itself be petted.
• Der Hund lässt ihn streicheln. = The dog lets him pet someone/something else.
  That would mean something very different.

So sich shows that the action comes back to the subject.

Because streicheln is the infinitive that goes with lässt.

In German, when you have a conjugated verb plus another verb in the infinitive, the infinitive usually goes to the end of the clause.

Here:

• lässt = the finite, conjugated verb
• streicheln = the infinitive

So the structure is:

Der Hund | lässt | sich von allen Kindern | streicheln

This is normal German word order.

allen Kindern is dative plural.

That is because it follows the preposition von, and von always takes the dative.

So:

• von
  • dative
• allen Kindern = by all the children

You can see the plural dative clearly here:

• alle Kinder = all children
• von allen Kindern = by all the children

Also notice the -n on Kindern, which is typical for many dative plural nouns.

von allen Kindern tells you who is doing the petting.

In English, this is like by all the children.

So the sentence means that the dog allows itself to be petted by all the children.

In German, von is commonly used to express the agent in passive-style meanings:

• Der Hund wird von allen Kindern gestreichelt.
• Der Hund lässt sich von allen Kindern streicheln.

Both include the idea by all the children, though the nuance is a little different.

Both sentences can be translated similarly, but they are not exactly the same.

Der Hund wird von allen Kindern gestreichelt is a regular passive:

• The dog is being petted by all the children.

It simply describes what is happening.

Der Hund lässt sich von allen Kindern streicheln adds the idea that the dog is willing, tolerant, or easy to pet. It suggests something about the dog's attitude or nature.

So:

• wird gestreichelt = focuses on the action
• lässt sich streicheln = focuses on the dog's readiness to allow it

lassen has several meanings in German, and that often confuses learners.

Some common meanings are:

• to let / allow
• to have something done
• to leave
• in sich lassen + infinitive: to be possible / to allow itself to be

In this sentence, it does not mean leave.

Here, lassen means something like allow:

• Der Hund lässt sich streicheln = The dog allows itself to be petted

So this is one of those verbs whose meaning depends a lot on the structure around it.

Because the main verb is lässt, not streicheln.

The dog is the subject of lässt:

• The dog allows ...

The children are the ones performing the petting, but that petting is inside the infinitive phrase streicheln.

So grammatically:

• Der Hund = subject in the nominative
• lässt = main finite verb
• sich = reflexive pronoun referring back to the dog
• von allen Kindern = tells us who pets the dog
• streicheln = infinitive at the end

This is why the sentence is built around the dog, not the children.

Not exactly.

It suggests that the dog is okay with it and does not resist, but it does not directly say the dog likes it.

So the idea is closer to:

• The dog lets all the children pet it
• The dog allows itself to be petted by all the children
• The dog can be petted by all the children

If you wanted to say the dog actively enjoys it, German would usually choose different wording, such as something with gern or a verb expressing enjoyment more directly.

Because alle Kinder would be nominative or accusative plural, but after von you need the dative.

So:

• alle Kinder = all children
• von allen Kindern = by all the children

This change affects both words:

• alle → allen
• Kinder → Kindern

That is a very common pattern in German case marking.

Yes. The finite verb lässt is the present tense form of lassen.

So the sentence means something like:

• The dog lets all the children pet it
• The dog allows itself to be petted by all the children

Depending on context, the present tense in German can describe:

• something happening now
• a general habit
• a typical characteristic

Here it most naturally sounds like a general fact about the dog.