Çocuk büyükbabasıyla yürüyüşe çıktı.

Because the most neutral Turkish word order is verb-final.

So this sentence is arranged like:

• Çocuk = subject
• büyükbabasıyla = with his/her grandfather
• yürüyüşe = for a walk
• çıktı = went out

A very literal structure would be:

• The child with-his-grandfather for-a-walk went-out

Turkish can change word order for emphasis, but putting the verb at the end is the default.

It breaks down like this:

• büyükbaba = grandfather
• -sı = his/her
• -yla = with

So:

• büyükbabasıyla = with his/her grandfather

The -yla part is a shortened attached form of ile, which also means with.

The -sı is a 3rd-person possessive suffix, so büyükbabası means his/her grandfather.

This is very natural with family words in Turkish. When talking about someone’s own family member, Turkish often uses a possessed form:

• annesi = his/her mother
• babası = his/her father
• dedesi / büyükbabası = his/her grandfather

So:

• çocuk büyükbabasıyla = the child with his/her own grandfather

If you said büyükbabayla, that would sound more like with a/the grandfather, not necessarily the child’s own grandfather.

From context.

The suffix -sı already gives his/her. In this sentence, the natural interpretation is that it refers back to çocuk.

So the sentence is understood as:

• The child went out for a walk with his/her grandfather.

Turkish often leaves this kind of relationship implicit if it is obvious from the sentence.

Because Turkish uses the expression yürüyüşe çıkmak for to go out for a walk.

Here:

• yürüyüş = walk, stroll
• -e = dative ending

So yürüyüşe çıkmak is literally something like:

• to go out to a walk

But idiomatically it means:

• to go out for a walk

This is one of those verb patterns you learn as a chunk.

Because yürüyüşe çıkmak is a very common Turkish expression.

• çıkmak basically means to go out / to exit
• gitmek means to go

In English, we say go for a walk.
In Turkish, the natural phrasing is often go out for a walk:

• yürüyüşe çıkmak

So this is not just a literal verb choice; it is a standard expression.

It is the simple past (also called the definite past) of çıkmak.

Breakdown:

• çık- = verb stem
• -tı = past tense ending

So:

• çıktı = went out

Why -tı and not -dı?
Because Turkish changes the first consonant of this suffix according to sound harmony. After the voiceless consonant k, the suffix appears as -tı.

Because Turkish does not have articles like English the and a/an.

So çocuk can mean:

• the child
• a child

Which one is intended depends on context.

In a sentence by itself, English might translate it either way. If the child is already known in the conversation, the child is more natural.

Yes.

Both are correct:

• büyükbabasıyla
• büyükbabası ile

The attached form is very common in everyday Turkish. The separate form with ile may feel a bit more formal or careful in writing, but the meaning is the same.

That y is a buffer consonant.

The word büyükbabası ends in a vowel sound, and ile also begins with a vowel, so Turkish inserts y when the short attached form is used:

• ile → -yla / -yle

So:

• büyükbabası + ile → büyükbabasıyla

This helps the word flow more smoothly.

Yes, Turkish word order is flexible.

For example, you could hear things like:

• Büyükbabasıyla çocuk yürüyüşe çıktı.
• Çocuk yürüyüşe büyükbabasıyla çıktı.

These versions shift the emphasis a little, but the basic meaning stays the same.
The original sentence is a very normal, neutral way to say it.

It is correct, but in everyday Turkish, dede is often more common.

So you may also hear:

• Çocuk dedesiyle yürüyüşe çıktı.

That means essentially the same thing.

Very roughly:

• büyükbaba = more explicit, a bit more formal/literal
• dede = very common in daily speech

Both are useful to know.